audioflinger: implement pause/resume for direct outputs

Extend pause/resume support to direct output threads
(was only for offload threads).

If the HAL implements pause/resume, track pause/resume is forwarded to
the HAL.

Pause, flush, resume sequence is respected by executing the HAL
calls in the playback thread (same as offload).

Make sure the track flags on client side are consistent with the
flags on server side.

Bug: 17883772.
Change-Id: I89b360d69818f7a9204bd36e3ec63a79e106ecf1