MediaPlayer: trigger onError instead of exception on stale native notifications

This change also reverted commit 2c838fbd87ad5685c0008b419ea02421159b9b70,
"MediaPlayer: add mPreparing to weed out unwanted prepared messages".

Bug: 21266735
Change-Id: I392342519420bee2fde1fe1915c51d24101ca27d
2 files changed