Clean up AudioTrack::mActive and stopped()

mActive is protected by mLock; volatile is meaningless on SMP.

Fixed a couple of places where mActive was accessed without a lock:
 - stopped()
 - processAudioBuffer()

Added stopped_l() for cases where we already hold the lock.

Made mActive a bool not int.

Moved down a lock in setPosition that was being acquired too early.

Change-Id: I73ff368e991c0db9f9472df0b3f96fd33fcc7311