Ensure fullscreen and immersive VR video is exited on pause (#3182)

Fixes #3154

Previously if the content process was killed or crashed while the
application was paused, the windows would still be in fullscreen
and/or immersive VR video mode while the recreated GeckoSession
would not. This partially fixes the issue by ensuring fullscreen
and VR immersive video is exited on pause so the windows do not
get into a bad state if the crash/kills while the application is
backgrounded.

app/src/common/shared/org/mozilla/vrbrowser/VRBrowserActivity.java

@@ -417,13 +417,30 @@ protected void onStop() {
         GleanMetricsService.sessionStop();
     }
 
+    public void flushBackHandlers() {
+        int backCount = mBackHandlers.size();
+        while (backCount > 0) {
+            mBackHandlers.getLast().run();
+            int newBackCount = mBackHandlers.size();
+            if (newBackCount == backCount) {
+                Log.e(LOGTAG, "Back counter is not decreasing,");
+                break;
+            }
+            backCount = newBackCount;
+        }
+    }
+
     @Override
     protected void onPause() {
         if (mIsPresentingImmersive) {
             // This needs to be sync to ensure that WebVR is correctly paused.
             // Also prevents a deadlock in onDestroy when the BrowserWidget is released.
             exitImmersiveSync();
         }
+        // If we are in fullscreen or immersive VR video, we need to consume their back handlers
+        // before pausing to prevent the windows from getting stuck in fullscreen mode.
+        flushBackHandlers();
+
         mAudioEngine.pauseEngine();
 
         mWindows.onPause();

app/src/common/shared/org/mozilla/vrbrowser/browser/engine/Session.java

@@ -1192,9 +1192,14 @@ public void onContextMenu(@NonNull GeckoSession session, int screenX, int screen
 
     @Override
     public void onCrash(@NonNull GeckoSession session) {
-        Log.e(LOGTAG,"Child crashed. Creating new session");
+        Log.e(LOGTAG,"Child crashed. Recreating session");
+        recreateSession();
+    }
+
+    @Override
+    public void onKill(@NonNull GeckoSession session) {
+        Log.e(LOGTAG,"Child killed. Recreating session");
         recreateSession();
-        loadUri(getHomeUri());
     }
 
     @Override