libstagefright: Fix ANR with rigorous rewind of 3gp LPA clips.

-ANR is seen with rigorous rewind of 3gp LPA clips.
-The lpa destructor waits for decoder thread to be killed when
 the thread function has not got the time-slice to execute after
 creation.
-Use mLock instead of extractor mutex. use mLock before signalling
 the thread to avoid indefinite wait.
CRs-Fixed: 491077 490992

Change-Id: Icf2ae64a379ff2b1260d34dd0e5c45f07f429718