Behavior compat for child being manipulated outside Animatorset

When a child animator is being manipulated outside of the AnimatorSet,
such as when the child animator is running before the AnimatorSet gets
started, we need to make sure we maintain the behavoir on N:
The child animator will be re-started at its scheduled start time in
the AnimatorSet.

BUG: 37507882
Test: Repro steps in comment #1 & ag/2144081/
Change-Id: Ib97e85706f01d18a2e72fb4d1d678e22cf959894