Simplify AudioTrack event handling and fix a race

Fixed race where mEventHandlerDelegate was accessed multiple times with
an assumption that all values read were the same.

If setPlaybackPositionUpdateListener was called with a null listener,
it did not update the delegate.

Inner classes can access final local variables of the enclosing method.
Use this to remove fields mPositionListener, mPositionListenerLock, mListener.

Change-Id: Ibf5cae61a1a9d21b745b90d4b74f9be14b66e37b