Trigger animation start after transaction applied...

for predictive back animation. Then shell can apply other transaction
effect on leashes without cause flicker.
The reparenting transaction for animation target to animation leash
is occur in WindowAnimator, so it might happen after core dispatch
leashes to shell.

Bug: 328446606
Test: In shell, attach a scrim layer on animation leash asa receive it,
verify no flicker occur(i.e. ag/26460881)

Change-Id: If61d95bbcd00837340365523dc2cf7826efe088e