Add swipe-to-dismiss notifications in popup menu.

- Next secondary icon animates up to replace dismissed main notification
- Add padding around main notification so it always aligns with the
  straight edges of the view (not the rounded corners); looks more
  dismissable
- Notification view collapses as notifications are dismissed
- To mimic system notification behavior, we copy SwipeHelper,
  FlingAnimationUtils, and Interpolators. We also apply elevation
  to notifications and reveal a darker color beneath when dismissing.

Bug: 32410600
Change-Id: I9fbf10e73bb4996f17ef061c856efb013967d972