audio policy service: fix potential deadlock in destructor

In the AudioCommandThread loop, release mLock before releasing
the strong reference on the service as AudioPolicyService destructor
calls AudioCommandThread::exit() which acquires mLock.

Also check exiPending in thread loop before processing pending
commands or sleeping.

Bug: 15449050.

Change-Id: I148bf21bd67ef721b5b5ee2c1a6afb185c59daa3