Ensure finish callbacks are called after all animations finish in SplitDecorManager

* Previously if a callback didn't get run because another animation was running
it got silently dropped instead of being saved to run after all animations finished
* Now save it into a map that all animations should call onAnimationEnd()

Testing: Manual, drag to enter, launch adjacent
Fixes: 420327582
Flag: EXEMPT bugfix
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:7ed6525446c04c18156534157f6489c973430c39)
Merged-In: I29c4ff514a5c92399e6521e660779a6ae6ebd920
Change-Id: I29c4ff514a5c92399e6521e660779a6ae6ebd920