Don't use sync transactions if not needed

Some common operations (like setting focusability) don't need
to be sync. In fact, sync is causing problems because it causes
other simultaneous operations (like transition animations) to
put some of their operations on the sync transaction instead
of pending. For example, leash-reparent would go on pending,
but activity hide would go on sync.

Additionally, the divider kept re-updating the app bounds
even when they didn't change. This added to the likelihood
that transactions would get mixed-up.

Bug: 158767094
Test: In split-screen landscape, quickly open/close apps in
      secondary split and look for flickers.
Change-Id: I0151e013347e9b30d58a52e8bd6b1401defee572