Handle cases for going to SHADE_LOCKED

- When the user drags down in the empty (not on a notification), we
  scale up the clock in the interaction.
- If there are no notifications at all, we stay in KEYGUARD and let
  the clock spring back.
- Rubberbanding is different for the case when there are no
  notifications or only low-priority notifications, to indicate what
  the user can expect when they lift his finger.

Bug: 16216907
Change-Id: I69d6552c96ef9e3d0d0b526bbd232d68bef47960