Fix several issues with RecentsAnimation

- Prevent starting the recents animation while an app transition is already
  set, if the animation was canceled before the pending start animation
  could be processed, or if there are truly no visible tasks to animate
- Fix case where we were improperly cleaning up the recents animation if it
  was canceled while initializing. In particular we need to move the
  initialization out of the constructor for the cancel logic to work
  (which checks for a non-null controller before cleaning up). Similarly
  we were posting the timeout runnable after the initialization of the
  recents animation, even if that resulted in the animation being canceled.
  Instead, post it before initialization to ensure that canceling the
  animation will also remove the timeout.

Bug: 72953248
Test: Introduce artificial delay when starting recents animation such that
      it happens after starting an activity

Change-Id: Ie0ec37038822f28327e789e7e5a3c4c36f945ff9
Signed-off-by: Winson Chung <winsonc@google.com>