Support for apps with multiple GVRActivities

[liaxim]

See https://github.com/Samsung/GearVRf/issues/1044; there have been others too.

Investigate the technical limitations that prevents us from properly supporting this. While it is not likely a good idea to switch back and forth between activities in VR, we could figure out the details and have the capability to do so. Changes to the backends are probably necessary.

[sam16222]

In case this helps,

The onInit() method of the corresponding GVRMain classes is not called after the first execution(checked via logs). But if you don't follow the activity stack and instead navigate to the main activity via an intent, the entire setup resets and then corresponding activities work.