Put VD animators on paused list when RT stops drawing

When we stop scheduling for new frames on RenderThread, we should
put the running animations on pause, rather than purge the list
of the running animations, such that in the next full
sync, the animations that were paused will continue to run.

BUG: 30226711
Change-Id: I36ff6f5d26ffa7999f60ca0ff676a35157577dc2