Assalamu Alaikum, friends

Welcome to BD Help.

Very easily android Application Full screen  Open. 
First watch the video.









AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.any_browse_support_downloading">

    <uses-permission android:name="android.permission.INTERNET"/>

    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>
MainActivity

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"

    tools:context=".MainActivity">


    <android.support.constraint.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent">

        <WebView
            android:id="@+id/mWebView"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:layout_alignParentStart="true"
            android:layout_alignParentTop="true"
            tools:ignore="RtlCompat" />

        <ProgressBar
            android:id="@+id/progressbar"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_centerInParent="true"
            android:layout_marginStart="8dp"
            android:layout_marginLeft="8dp"
            android:layout_marginTop="8dp"
            android:layout_marginEnd="8dp"
            android:layout_marginRight="8dp"
            android:layout_marginBottom="8dp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />





    </android.support.constraint.ConstraintLayout>




</RelativeLayout>
MainActivity.java
package com.example.any_browse_support_downloading;

import android.annotation.SuppressLint;
import android.app.DownloadManager;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.net.Uri;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.webkit.DownloadListener;
import android.webkit.WebChromeClient;
import android.webkit.WebSettings;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.FrameLayout;
import android.widget.ProgressBar;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {




    WebView mWebView;
    ProgressBar progressBar;



    @SuppressLint("SetJavaScriptEnabled")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        progressBar = (ProgressBar) findViewById(R.id.progressbar);
        mWebView = (WebView) findViewById(R.id.mWebView);

        progressBar.setVisibility(View.VISIBLE);
        mWebView.setWebViewClient(new MainActivity.Browser_home());
        mWebView.setWebChromeClient(new MainActivity.MyChrome());
        WebSettings webSettings = mWebView.getSettings();
        mWebView.getSettings().setJavaScriptEnabled(true);
        mWebView.getSettings().setLoadWithOverviewMode(true);
        mWebView.getSettings().setUseWideViewPort(true);

        mWebView.getSettings().setSupportZoom(true);
        mWebView.getSettings().setBuiltInZoomControls(true);
        mWebView.getSettings().setDisplayZoomControls(false);

        mWebView.setScrollBarStyle(WebView.SCROLLBARS_OUTSIDE_OVERLAY);
        mWebView.setScrollbarFadingEnabled(false);
        webSettings.setJavaScriptEnabled(true);
        webSettings.setAllowFileAccess(true);
        webSettings.setAppCacheEnabled(true);
        loadWebsite();



    }

    private void loadWebsite() {
        ConnectivityManager cm = (ConnectivityManager) getApplication().getSystemService(Context.CONNECTIVITY_SERVICE);
        NetworkInfo netInfo = cm.getActiveNetworkInfo();
        if (netInfo != null && netInfo.isConnectedOrConnecting()) {
            mWebView.loadUrl("https://www.youtube.com/");
        } else {
            mWebView.setVisibility(View.GONE);
        }

        mWebView.setDownloadListener(new DownloadListener() {
            @Override
            public void onDownloadStart(String url, String userAgent, String contentDisposition, String mimetype, long contentLength) {

                DownloadManager.Request myRequest = new DownloadManager.Request(Uri.parse(url));
                myRequest.allowScanningByMediaScanner();
                myRequest.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE_NOTIFY_COMPLETED);

                DownloadManager myManager = (DownloadManager) getSystemService(DOWNLOAD_SERVICE);
                myManager.enqueue(myRequest);

                Toast.makeText(MainActivity.this,"Your file is downloading...", Toast.LENGTH_SHORT).show();


            }
        });

    }

    class Browser_home extends WebViewClient {

        Browser_home() {
        }

        @Override
        public void onPageStarted(WebView view, String url, Bitmap favicon) {
            super.onPageStarted(view, url, favicon);

        }

        @Override
        public void onPageFinished(WebView view, String url) {
            setTitle(view.getTitle());
            progressBar.setVisibility(View.GONE);
            super.onPageFinished(view, url);

        }
    }

    private class MyChrome extends WebChromeClient {

        private View mCustomView;
        private WebChromeClient.CustomViewCallback mCustomViewCallback;
        protected FrameLayout mFullscreenContainer;
        private int mOriginalOrientation;
        private int mOriginalSystemUiVisibility;

        MyChrome() {}

        public Bitmap getDefaultVideoPoster()
        {
            if (mCustomView == null) {
                return null;
            }
            return BitmapFactory.decodeResource(getApplicationContext().getResources(), 2130837573);
        }

        public void onHideCustomView()
        {
            ((FrameLayout)getWindow().getDecorView()).removeView(this.mCustomView);
            this.mCustomView = null;
            getWindow().getDecorView().setSystemUiVisibility(this.mOriginalSystemUiVisibility);
            setRequestedOrientation(this.mOriginalOrientation);
            this.mCustomViewCallback.onCustomViewHidden();
            this.mCustomViewCallback = null;
        }

        public void onShowCustomView(View paramView, WebChromeClient.CustomViewCallback paramCustomViewCallback)
        {
            if (this.mCustomView != null)
            {
                onHideCustomView();
                return;
            }
            this.mCustomView = paramView;
            this.mOriginalSystemUiVisibility = getWindow().getDecorView().getSystemUiVisibility();
            this.mOriginalOrientation = getRequestedOrientation();
            this.mCustomViewCallback = paramCustomViewCallback;
            ((FrameLayout)getWindow().getDecorView()).addView(this.mCustomView, new FrameLayout.LayoutParams(-1, -1));
            getWindow().getDecorView().setSystemUiVisibility(3846);
        }
    }

}

Android Webview make back button go to previous page


 public boolean onKeyDown(int keyCode, KeyEvent event) {

        if (event.getAction() == KeyEvent.ACTION_DOWN ){


            switch (keyCode) {

                case KeyEvent.KEYCODE_BACK:

                    if (mWebView.canGoBack()) {

                        mWebView.goBack();
                    }else {

                        finish();
                    }

                    return true;            }


        }

        return super.onKeyDown(keyCode,event);    }
}