Fixing crash when focusing tasks.

- We call bindVisibleTaskViews() at the new scroll value expecting all
  visible task views at that scroll to be bound synchronously, however, 
  the call to computeVisibleTaskTransforms() would not produce the 
  correct visible range if the visible range at the current and new
  scroll resulted in gaps (ie. at scroll 0, 0-3 are visible, and at 
  scroll s, 5-8 were visible).  As a result, the task view would not be
  bound, causing a later crash.  

  Instead of the optimization before, which would assume every task 
  after the first non-visible task would also be non-visible, we can
  just update the transforms for all task views.  This ensures that 
  all visible task views should be bound, even if they are not in a 
  contiguous range.  A global range is still provided in the call to 
  allow accessibility to focus the next task when a task is unbound at 
  the two ends of the stack.
- Also removing expensive calls to remove all listeners and cancel 
  animations that are not even started.

Bug: 27295423

Change-Id: Ib309e9de3095cacb178ae9ff5c53b8ee2c7ddac0