Cancel running split decor animations before finishing the current resize transition

- One possible flow for this:
  1) play a pending split resize animation with a transition that
     includes a snapshot -> this creates a SplitDecorManager animation
     which directly animates the snapshot leash
  2) have another resize which triggers setup of a new pending resize
     animation -> this cancels the current animation which cleans up/
     releases the existing transition's surfaces, but nothing notifies
     SplitDecorManager, so that animation continues to play
  3) Next frame comes in, SplitDecorManager's animation tries to update
     the surface that has already been released -> crash
- This change just cancels any existing animations (which will remove
  the decor) before creating a new pending resize animation and
  finishing the previous transition

Fixes: 325306101
Test: Presubmit (not reproducible)
Change-Id: Id77f02871bfd684e8dd814b3a38a94e06049e210