Recents: apps scale down to thumbnails now

As a part of this change, Recents is now an
activity.

Known issues:
* Jank: jump-cut as app icon appears suddenly
  after the aniamtion
* Preloading recents is broken on phones without
  soft nav bar and on tablets
* Thumbnail window from animation lingers/flashes
  sometimes

Change-Id: Ie6f991f3c2e1e67f9ed84eb6adba9174ed957248