Keep recents attached to app window on continued switching

- LAST_TASK now has recentsAttachedToAppWindow = true, to match
  NEW_TASK. Though this does mean there's a small chance of
  seeing recents translating in if you swipe up and back down,
  this is preferable to seeing recents translate away if you
  swipe right and back left in two consecutive gestures.
- Until passing the touch slop for a new gesture, keep recents
  attached to the app window. This avoids cases where you
  could have recents view translate away mid-gesture, such as
  in the following case: swipe from A to B, swipe from B back
  to A, touch down during the transition back to A. Before this
  change, you would see recents translate away; with this
  change, recents instead stays in place until we get another
  signal indicating it should detach, such as swiping to home.

Bug: 157153404
Change-Id: I63a3a8b988af1aac5177f101b0b75e73e56b1ceb