AudioFlinger: Fix direct output resource from being occupied for too long

When flush immediately follows pause, mState might be overwritten and not
processed. In this case, direct output will retry due to data underrun.
In the duration of retry, the ActiveCount in AudioPolicy will be
positive and user can't create new track.
Add isPausePending to make sure pause will be handled by the
direct output thread.

Test: Manual (see bug)
Test: Clarity MediaPlayer Offloaded with seek.
Bug: 175092336
Change-Id: Ief742d2ce44917da24885373a5770e47876bc3a5