(Android) Big rewrite -
* app now starts with RetroActivity -
RetroActivity in turn can call MainMenuActivity
* You can switch back and forth between RetroActivity and
MainMenuActivity by pressing AKEYCODE_BACK
* MainMenuActivity no longer fires off intents to start
RetroActivity - instead, it sets static methods inside RetroActivity
* RetroActivity keeps running in the background and when switching
back to RetroActivity, it will do a JNI call to 'onPendingStateChanges'
- if it returns true, it indicates to RetroActivity that it has a
pending state change waiting for it that was set by MainMenuActivity.
This can be updating the libretro core path, updating the ROM to be
played (and ran), and/or updating IME (will be done later on)

Things that are glitchy right now and will need fixing:
* History list needs to be more consistently updated between
RetroActivity and MainMenuActivity.
* We need to flag when we need RetroActivity to reload the config
file based on what was changed in MainMenuActivity. I can do this later
on.
* Libretro GL cores currently have some issues with switching back
and forth between MainMenuActivity and RetroActivity - GL context is
probably being lost