libstagefright: Release mLock before TimedEventQueue stop

During suspend call, mQueue.Stop() gets blocked for onVideoEvent()
to complete, which in turn tries to acquire the mLock held
by suspend(). Thus, it leads to deadlock.

Changes are done to release the lock before calling mQueue.Stop()
and acquire the lock once stop is finished.

Change-Id: Iadb14299bbdd0ae7e80baffc273a910c8d6ff9bb
CRs-Fixed: 730002