Making stack visibility more specific to the launching tasks.

- When going from freeform -> overview, hide all freeform tasks until
  the final frame of the enter transition, if the stack we are
  launching to/from is in the freeform stack
- When going from overview -> freeform, hide all freeform tasks after
  the first frame of the exit transition, if we are launching a task
  in the freeform stack
- Separate the dragging gesture from the freeform launch visibility

Bug: 25842568

Change-Id: Ibcd17a02eeb70244ac73f599b8c23c71d22f73fd