Expand notifications like accordian

Show all notifications in the same section at the same time.

On each invocation of StackScrollAlgorithm
- compute fraction of section showing based on current expansion amount
- apply that fraction to each view's intrinsic height

The notification icon shelf no longer slides down from top of screen,
instead it shows if the notification section before it is showing.

Bug: 172289889

[Test accordian effect]
Test: open shade with single section, no shelf
Test: open shade with single section, silent section, no shelf
Test: open shade with first section overflowing into shelf
Test: open shade with first section, silent section overflowing into shelf
Test: open shade with no notifications (empty shade view)
Test: open shade with shelf, scroll notifications, then close shade
  => accordian effect applies to scrolled state

[Test for regressions]
Test: add/remove delayed notification that arrives while shade opens
  => animation instantly updates for new notifs (while expansion runs)
Test: open shade from heads up notification
Test: open shade from pulsing (incoming notif on aod)
Test: open shade from lockscreen

Change-Id: If3236b9dc202ee75db7cac51a66c49620556ec10