Fix miscellanous AudioTrack::getTimestamp() bugs

Check that get_presentation_position is non-NULL before calling.

AudioTrack::getTimestamp not implemented for fast tracks.

Fix typo in Track::getTimestamp().

Fix bugs in AudioTrack::getTimestamp after stop:
 - getTimestamp while stopped is not allowed.
 - stop, start, getTimestamp now returns the correct value.

Change-Id: Ie8d9dc1f28d8927634e04175a68b147ffc2ea8eb