audioflinger: fix duplicating thread standby

The code that waits for a track presentation to be complete
before disabling it caused a regression for duplicating threads.
Because of the way output tracks activity is managed, the number
of frames output by the duplicating thread would never
reach the target set for a track to be considered presented.
The track would not be removed from active list and the thread would
not go to standby and keep its wakelock held.

Bug 6606922.

Change-Id: I4b46b420ac4cbf79a86b6791ae6589d407b01c92