Fix a couple of issues with quick switch from home to empty recents

- Add SKIP_SCRIM for mNonOverviewAnim, as the scrim  is already
  handled by mXOverviewAnim (this prevents some flickering).
- If RecentsView is empty, fade it in. This is less jarring than
  the "No recent items" appearing immediately without animation,
  and ensures it fades out seamlessly if you return home instead.

Test: Quick switch from home with no tasks and 1 task
Bug: 174228745
Change-Id: I57edc0cea4d1dfa40f87f7ccc5aa3d6b8929a1ff