AndroidSound#play/loop now returns -1 on failure, to match other backends

There is currently an inconsistency in the backends; OpenALSound#play and IOSSound#play both return -1 on failure, while AndroidSound returns 0. This is because of the Android [SoundPool](http://developer.android.com/reference/android/media/SoundPool.html#play(int, float, float, int, int, float\)).
This fix simply modifies AndroidSound to match the other backends.