[Android] Implement DefaultHardwareBackBtnHandler

If the JS side chooses to not handle the back button press call
'super.onBackPressed()' to close the app.

android/sdk/src/main/java/org/jitsi/meet/sdk/JitsiMeetActivity.java

 
 import java.net.URL;
 
+import com.facebook.react.modules.core.DefaultHardwareBackBtnHandler;
+
 /**
  * Base Activity for applications integrating Jitsi Meet at a higher level. It
  * contains all the required wiring between the <tt>JKConferenceView</tt> and
  * hooked to the React Native subsystem via proxy calls through the
  * <tt>JKConferenceView</tt> static methods.
  */
-public class JitsiMeetActivity extends AppCompatActivity {
+public class JitsiMeetActivity
+    extends AppCompatActivity
+    implements DefaultHardwareBackBtnHandler {
+
     /**
      * The request code identifying requests for the permission to draw on top
-     * of other apps. The value must be 16-bit and is arbitrarily chosen here. 
+     * of other apps. The value must be 16-bit and is arbitrarily chosen here.
      */
     private static final int OVERLAY_PERMISSION_REQUEST_CODE
         = (int) (Math.random() * Short.MAX_VALUE);
         }
     }
 
+    /**
+     * This method is called if the JS part does not handle the physical back
+     * button press.
+     */
+    @Override
+    public void invokeDefaultOnBackPressed() {
+        super.onBackPressed();
+    }
+
     /**
      * {@inheritDoc}
      */
     protected void onResume() {
         super.onResume();
 
-        JitsiMeetView.onHostResume(this);
+        JitsiMeetView.onHostResume(this, this);
     }
 
     /**

android/sdk/src/main/java/org/jitsi/meet/sdk/JitsiMeetView.java

 import android.widget.FrameLayout;
 
 import com.facebook.react.ReactInstanceManager;
+import com.facebook.react.ReactInstanceManagerBuilder;
 import com.facebook.react.ReactRootView;
 import com.facebook.react.bridge.NativeModule;
 import com.facebook.react.bridge.ReactApplicationContext;
 import com.facebook.react.common.LifecycleState;
+import com.facebook.react.modules.core.DefaultHardwareBackBtnHandler;
 
 import java.net.URL;
 import java.util.Arrays;
      * <tt>Activity.onResume</tt> so we can do the required internal processing.
      *
      * @param activity - <tt>Activity</tt> being resumed.
+     * @param backHandler - a <tt>DefaultHardwareBackBtnHandler</tt> which
+     * will handle the back button press in case there's nothing to handle on
+     * the JS side.
      */
-    public static void onHostResume(Activity activity) {
+    public static void onHostResume(Activity activity,
+                                    DefaultHardwareBackBtnHandler backHandler) {
         if (reactInstanceManager != null) {
-            reactInstanceManager.onHostResume(activity, null);
+            reactInstanceManager.onHostResume(activity, backHandler);
         }
     }