Start dim animations on pending vs sync transaction

We try to put animations on pending transaction to avoid
jank. In this case, it also avoids bad state (where a surface
is created/parented after its used for animation and
potentially cleaned-up).

Bug: 159336447
Test: enter split primary, in recents, take a screenshot and choose
      share, then expand the primary to leave split. Observe no
      lingering dim layer.
Change-Id: Ia1301a3b22c6eb07192c99b2bfd9aa60ed3c7e95