New shelf shape, notification corner animations on scroll

Bug: 182486170
Fixes: 186168632

Test: visual
- shelf shape stays constant (top/bottom corners stay round)
- notification scrolling up from shelf first shows round
  top/bottom corners, then "merges" top corners with notification
  above if in same section; vice versa happens for scroll down
- on lockscreen, notif before shelf has round bottom corners
- no change while dozing/pulsing

Change-Id: I1ef915e3b91cb16e6277f8eaa1bff63dcc425dde