Part 1: Tweaking history transition to match design.

- Adding background scrim that can be controlled across transitions 
  within recents.  As a result, we can remove the status bar scrim.
- Moving the history view into the RecentsView now that it animates in
  parallel with the task stack
- Transition home from history no longer goes back to stack view first
- Removing some extra allocations when going into history and loading 
  tasks

Change-Id: I665baefcdd619de5e9366923eaaf4c558261141a