Fix multi-window drag jank if vsync-app is before vsync-sf

If vsync-app is a couple of ms before vsync-sf, we handle the touch
in SystemUI at vsync-app. Then, we have extremely little time to resize
all the stacks in activity manager until vsync-sf, so sometimes this is
done before vsync-sf tick, and sometimes after, which leads to jank.

Figure out the difference of vsync-app and vsync-sf in SystemUI and then
post the updates to be processed at vsync-sf so we have the whole 16ms
to process the change.

Change-Id: Ibe7cb6dfe9fbfe2d3e68e522a95a75138fb0dcf1
Fixes: 30437123