ViewRootImpl: Pass buffer to finishDrawing when requested

As the final enabler for the BLASTSyncEngine, we add a new relayout
result BLAST_SYNC. If the WindowState is participating in a BLAST sync
during relayout we return it to the client. If we return it to the
client the client will direct it's next draw in to a BLAST transaction
and report that transaction via finishDrawing. You can now observe
the BLASTSyncEngine working end to end in TaskOrganizerMultiWindowTests.
We do a few small clean-ups in TaskOrganizerMultiWindowTest while we
are there.

Bug: 153561718
Test: TaskOrganizerMultiWindowTests
Change-Id: I719b731350b942aafa444a33972aaef8973422ea