How to long press on my app based webview

Hi guy this is my first application android, i using android studio and i need the help. i long press in not show context menu to download the image, below I leave the code if someone is kind enough to give me help.

Code:

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import android.content.Context;
import android.media.AudioManager;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.KeyEvent;
import android.view.View;
import android.webkit.WebChromeClient;
import android.webkit.WebView;
import android.webkit.WebViewClient;
import android.widget.ProgressBar;

public class MainActivity extends AppCompatActivity {
    WebView webView;
    ProgressBar progressBar;
    private AudioManager audio;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        audio = (AudioManager) getSystemService(Context.AUDIO_SERVICE);

        webView = (WebView) findViewById(R.id.webView);

        webView.setWebViewClient(new WebViewClient());

        webView.getSettings().setSupportZoom(true);
        webView.setVerticalScrollBarEnabled(true);
        webView.setHorizontalScrollBarEnabled(true);
        webView.getSettings().setLoadWithOverviewMode(true);
        webView.getSettings().setUseWideViewPort(true);
        //webView.getSettings().setBuiltInZoomControls(true);
        webView.loadUrl("https://alpha.wallhaven.cc/");



        //Barra de progreso
        progressBar = (ProgressBar) findViewById(R.id.progressBar);
        webView.setWebChromeClient(new WebChromeClient() {
            @Override
            public void onProgressChanged(WebView view, int progress) {
                progressBar.setProgress(0);
                progressBar.setVisibility(View.VISIBLE);
                MainActivity.this.setProgress(progress * 1000);
                progressBar.incrementProgressBy(progress);
                if (progress == 100) {
                    progressBar.setVisibility(View.GONE);
                }
            }
        });


        /*Adding shortcut for MainActivity
        //on Home screen
        Intent shortcutIntent = new Intent(getApplicationContext(),
                MainActivity.class);

        shortcutIntent.setAction(Intent.ACTION_MAIN);

        Intent addIntent = new Intent();
        addIntent.putExtra(Intent.EXTRA_SHORTCUT_INTENT, shortcutIntent);
        addIntent.putExtra(Intent.EXTRA_SHORTCUT_NAME, "HelloWorldShortcut");
        addIntent.putExtra(Intent.EXTRA_SHORTCUT_ICON_RESOURCE,
                Intent.ShortcutIconResource.fromContext(getApplicationContext(),
                        R.drawable.ic_launcher));
        addIntent.putExtra("duplicate", false);
        addIntent.setAction("com.android.launcher.action.INSTALL_SHORTCUT");
        getApplicationContext().sendBroadcast(addIntent);*/

    }






    @Override
    public boolean onKeyDown(int keyCode, KeyEvent event) {

        switch (keyCode) {
            case KeyEvent.KEYCODE_VOLUME_UP:
                audio.adjustStreamVolume(AudioManager.STREAM_MUSIC,
                        AudioManager.ADJUST_RAISE, AudioManager.FLAG_SHOW_UI);
                return true;
            case KeyEvent.KEYCODE_VOLUME_DOWN:
                audio.adjustStreamVolume(AudioManager.STREAM_MUSIC,
                        AudioManager.ADJUST_LOWER, AudioManager.FLAG_SHOW_UI);
                return true;
            case KeyEvent.KEYCODE_BACK:
                if (webView.canGoBack()) {
                    //if Back key pressed and webview can navigate to previous page
                    webView.goBack();
                    // go back to previous page
                    return true;
                } else {
                    finish();
                    // finish the activity
                }
                return super.onKeyDown(keyCode, event);
            default:
                return false;
        }


    }



}

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