Ensure the animation finish sequence by synchronize call.

By making IBackAnimationFinishedCallback synchronize call, the client
app won't have chance to create a transition before core receives
onAnimationFinished, which could play the second close animation from
that transition.

Note: Shell side change can be found in ag/20459278.

Bug: 131727607
Test: monitor client won't create close transition before core handled
onAnimationFinished.

Change-Id: Iad3c70c14727d438633d804a573000f8b71b023e