Coordinating empty view with the history view.

- Moving the empty view into the RecentsView so
  that we can coordinate its animation with the
  history button and the task stack (when history
  is visible, all of the other views are animated
  away, and vice versa)
- Removing unnecessary launch state flag to keep
  track of recent task empty state just for deciding
  animations for system bar scrims.
- Fixing issue with overview button not dismissing
  the history view while it is open
- Fixing issue with swiping the last recent task
  away causing both Overview and the docked task
  to be dismissed to home

Bug: 26044055
Change-Id: I731fb0f7efb3de7d5f826673479c602b606453e9