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

core/java/android/window/IBackAnimationFinishedCallback.aidl

@@ -22,6 +22,6 @@ package android.window;
  * @param trigger Whether the back gesture has passed the triggering threshold.
  * {@hide}
  */
-oneway interface IBackAnimationFinishedCallback {
+interface IBackAnimationFinishedCallback {
     void onAnimationFinished(in boolean triggerBack);
 }