AudioFlinger: fix duplicating thread standby.

Commit fd477974 fixing issue 11247103 introduced a regression
where the duplicating thread does not exit standby state as
expected. This in turn prevents the tracks sending audio to
other threads to complete their activity cycle normally.

The fix conists in clearing mStandby state also in the specialized
threadLoop_write() method of DuplicatingThread subclass.

Bug: 11451415.
Change-Id: Ie8a92c819a56a2834ea25fa70a8b7b2125721775