Fix media player decode function. Bug 2317821.

There was a recent change to OpenCORE that broke the MediaPlayer::decode()
function for all OpenCORE supported formats (MP3, AAC, and AMR-NB/WB). The
affect is that SoundPool is unreliable for all of these formats. This patch
changes the logic in the AudioCache object that processes messages from the
media player. The old logic was fragile. This change should make it more
robust against changes.