AudioFlinger: Remove unwanted update of track generation

PlaybackThread::threadLoop unncessarily updates the track generation
before waiting for a pending aync write. This is unncessary and
leads to excess calls to the power manager service for updating
uids.

Change-Id: I1284d79da3be6495acb19a97c81b3ab8db6dbb12
(cherry picked from commit f10fdf3ac4aced7d64bd0d1f05cc886271436685)
(cherry picked from commit 48568df4edf4d9b98c4d05bcfae3770c91f3be7f)