Fix deadlocks in game-activity backend if java callbacks are fired after native thread exits

[rib]

See the discussion here for more context: #94 (comment)

Essentially the GameActivity backend doesn't have checks within its Java Activity callbacks that check whether the android_app has already been destroyed (e.g. due to a graceful exit by returning from android_main

Since android-activity will issue a GameActivity_finish when android_main returns that will result in onPause -> onStop -> onDestroyed lifecycled events in Java after the native thread for android_main has exit and then because the GameActivity callbacks don't check if the android_app is detroyed they deadlock waiting for the native thread to handle these lifecycle events.