Remove dead mutex in AudioTrack/AudioRecord thread

The client callback threads had mutexes called AudioTrackThread::mLock
and ClientRecordThread::mLock.  These mutexes were only used by start()
and stop(), and were unused by the thread itself.  But start() and
stop() already have their own protection provided by AudioTrack::mLock
and AudioRecord::mLock.  So the thread mutexes can be removed.

Change-Id: I098406d381645d77fba06a15511e179a327848ef