Clear video pause tracker when video un-pause reported by the modem.

When video calls are swapped, the video stream automatically unpauses
without the UI requesting the pause/resume. Thus, the video pause tracker,
which tracks pause requests from the UI, can get into a state where it
thinks the video is paused, do it will ignore further pause requests.

Test: Manual
Bug: 62779713
Bug: 62784036
Change-Id: Ia8add452dbe9a79957e8a49e20ce58317136403b