MediaActionSound: fix SoundPool load race condition

If play() was called shortly after load() and before the load
completion callback, the sample was considered
loaded although the SoundPool was not ready to play it.

Fixed by implementing a more robust state machine for sound
load and play control.

Bug: 22499793
Change-Id: I727e07f842a0adc5225dc248f16bc4a7225a1c8e