[Vision Glass] Back button when VR is not active

Fix the issue that the Back button on the phone is not responsive
before we have entered VR mode.

The fix is to add a method onBackPressed() to PlatformActivityPlugin
which returns true if the platform activity has handled the Back press.

In the Vision Glass implementation, we use the default Back behaviour
if the user presses that button before we have started VR mode.

app/src/common/shared/com/igalia/wolvic/PlatformActivityPlugin.java

@@ -9,6 +9,7 @@ public interface PlatformActivityPluginListener {
     }
     abstract void onKeyboardVisibilityChange(boolean isVisible);
     abstract void onVideoAvailabilityChange();
+    abstract boolean onBackPressed();
     void registerListener(PlatformActivityPluginListener listener) {
         if (mListeners == null)
             mListeners = new ArrayList<>();

app/src/common/shared/com/igalia/wolvic/VRBrowserActivity.java

@@ -997,6 +997,9 @@ private void showAppExitDialog() {
     @Override
     @Deprecated
     public void onBackPressed() {
+        if (mPlatformPlugin != null && mPlatformPlugin.onBackPressed()) {
+            return;
+        }
         if (mIsPresentingImmersive) {
             queueRunnable(this::exitImmersiveNative);
             return;

app/src/visionglass/java/com/igalia/wolvic/PlatformActivity.java

@@ -675,6 +675,16 @@ public void onLongPress(MotionEvent e) {
                 return mGestureDetector.onTouchEvent(event);
             });
         }
+
+        @Override
+        boolean onBackPressed() {
+            // User pressed Back on the phone and VR is not active, so we use the default behaviour.
+            if (mViewModel.getConnectionState().getValue() != PhoneUIViewModel.ConnectionState.ACTIVE) {
+                PlatformActivity.super.onBackPressed();
+                return true;
+            }
+            return false;
+        }
     }
 
     private final DisplayManager.DisplayListener mDisplayListener =