Starting the dismiss animation in parallel with the gesture.

- Introduces notion of ignored tasks for the purposes of layout in 
  TaskStackView.  This can be used during drag and drop, and while 
  dismissing to calculate the state of the stack without the task that
  the user is currently interacting with.
- Fixing minor layout issue when the front/back task transforms are 
  improperly calculated when there is a single task
- Fixing minor issue when the anchor task is calculated incorrectly when
  dismissing task views

Change-Id: I1eb0864a52e53562e4d573a6ed4f8a5a1615aff9