GameActivity backend should track lifetime of *mut android_app pointer more-carefully #235

The same issues with tracking the *mut ANativeActivity pointer lifetime in the native-activity backend (#234) also apply to how the game-activity backend currently tracks the *mut android_app state pointer.

The current implementation works OK if it's generally assumed that an application exits before or immediately-after receiving a MainEvent::Destroy event, but if an application were to retain an AndroidApp after the activity has been destroyed and then try to query state from the activity then that would currently result in a use-after-free when the backend dereferences the *mut android_app pointer after it has been freed.