Snap to running task if it's offscreen when back is invoked

- If the user has scrolled the running task offscreen, then
  snap back to the task if back is invoked.

Bug: 233952141
Test: Swipe to overview, scroll away from the current task
      and then go back
Change-Id: Icc5040d4b38c97e0329f937e24bd517a71a32f13