frameworks/av: Avoid timestamp query to AudioPlayer during reset

-Timestamp query while in reset leads to ANR in few cases.
-Destruction of mutex in AudioTrack destructor during reset and
trying to lock the destroyed mutex as part of getPosition() call
in parallel leads to ANR.
-Avoid querying for timestamp in case reset operation is in progress.
Instead use mAudioTearDownPosition to retrieve current playback
position.

Change-Id: Ifaa7f6236ebce1b27a0cca621bbb188c94af142d