Don't remove all animation callbacks if action/who was null.

Fixes a bug in View.removeCallbacks and View.unscheduleDrawable
where it was possible for the caller to remove all animation
callbacks if it happened to specify an action or who parameter
of null.

Also refactored the callback queueing code in Choreographer
to make it more intent revealing although the behavior remains
the same.

Bug: 6144688
Change-Id: Iba29dcda6b3aaad73af664bb63feab65ae3483e5