avsync-lib: support pause pts and step [1/3]

PD#SWPL-35128

The flow for step is:
1) set_pause_pts(AV_SYNC_STEP_PAUSE_PTS)
2) pop_frame() will pop next frame in queue.
3) callback() triggered.
Repeat to pop single frame.

The flow for pause is:
1) set_pause_pts(pts)
2) keep calling pop_frame() until pause pts is reached.
3) callback() triggerd.
4) pause(false) is called to resume.

Change-Id: Ib080b511eb3790bef3e9a4a66b8f1a4213d8f872
2 files changed