Don't prematurely finish sync if an explicit sync is in flight

If there is an explicit BLAST sync, we must wait for it even if
the window has already supposedly drawn. Otherwise an explicit
sync will be finished before it has the correct buffer.

Bug: 329154913
Test: enter pip via swipe + pip2, observe no content flicker
Change-Id: Ifac82f6b9be9f1228ce44509ff6786fc7ba09202