Uses applySyncTransaction to synchronize all the surface
updates during split-screen enter/exit.
In Divider, all syncTransactions are serialized and runnables
can be posted to operate on the returned transactions so
they happen all at once.
This also required adding a "merge" functionality on
WindowContainerTransactions to handle racing between rotation
and other in-flight sync transactions (because they are
serialized, sometimes they got delayed).
Bug: 153579514
Test: Enter/exit split-screen and observe no flickers
Change-Id: I57df13489247f3c4fb3ca59efa26867a872e447f
3613854d1b