Remove background drawing behind notifications

- NSSL: remove continuous background clipping, drawing, animating
- NotificationSection: remove bounds math
- AmbientState: remove unused state

Bug: 171817112
Test: background clear for new and old dismissal
Test: half-swipe, full-swipe, longpress on ls/hun/shade notifs; close
and open shade from ls and home; open shade after hun; no regressions

Change-Id: I9d7cb245542a03a526808ad8ed28763719ed4972