Fix state/anim clobbering when touching down during recents animation

Addresses 2 problems:
- Set mLauncherTransitionController = null after ending it, to prevent a
  subsequent end from re-setting OVERVIEW state.
- Cancel the shelf animation so that it doesn't clobber the animation to
  all apps (for instance).

Bug: 134309036
Bug: 134309189
Change-Id: I019084a1ba38c6208a6256843cb5c4b6dbb4a662