Fix issue 3302649.

The cause of the problem is that AudioTrack::start() can fail if it is called from a newly created
thread that has the same ID as the AudioTrack callback thread that has just been stopped and not yet exited.
This is possible as the thread ID used by the Thread class is not the TID.

The fix consists in clearing the thread ID before exiting the thread loop.

Change-Id: I8b5f6a63feeaeb9a01267380e85f6f1456e7aa01