libstagefright: Decrease reference count before deleting AudioPlayer

-Reference count of AudioSource is not decreased to 1 before deleting
 AudioPlayer, and thus cause the thread blocked in an infinite loop
 due to the AudioSource object is not deleted
-Decrease the strong reference value of AudioSource before
 deleting AudioPlayer

Change-Id: I804211dcca91213c62a4d622baf2001f3cc9955b
CRs-Fixed: 691668