AudioPolicyService: fix race in AudioCommandThread

Fixe race condition in AudioCommandThread::threadLoop() where a
command can be inserted in first position in the queue after the sleep
time has been calculated causing a longer delay than expected.

Also fix a failure to hold a wake lock while commands are still in the queue.

Bug: 22707905.
Change-Id: I813626986677bf00106acb37ee20d3dd75d5cf33