Don't clear sync information if WS is waiting on child windows

If a WindowState is waiting on child windows to draw, then we shouldn't
clear the sync info since it won't call onTransactionReady yet. When the
child window is done drawing, it will call onTransactionReady for the
parent WindowState and clear the sync information at that point.

Test: Split screen no longer crashes with child window
Fixes: 158007547
Change-Id: I301899db648aac6977a9e415a199143d34bd93a1