audio - Query midi decoder state after stop()

 - On midiplayer stop, mState was not updated correctly
   hence when isPlaying() returns true to application.
 - Fix is to query midi decoder state after stop() so that,
   isPlaying() will return false after midiplayer.stop() to
   application.

Change-Id: If1e260e93da1712873b8419b611abe3206dd0f47
CRs-Fixed: 633774