Fixes several animation issues related to a dismissing task.

- Previously we did not properly cancel the animations when the user 
  interacted with the stack while a task was being dismissed.  Now, on 
  touch down, we only cancel the non-dismissing tasks, and in doing so,
  set their task override if there is a dismissing task (so that the 
  stack can scroll correctly), and update the surrounding task views
  if the dismiss update comes from the currently dismissing task.  
  (or in other words, the surrounding task animations will only be a 
  function of the current task being dismissed)

Bug: 28448670
Change-Id: If7b8ed3f1fe342114ae0f6dd9dbff0d48204f31e