Remove half-swipe shelf behind notifications

Bug: 171817112

Test: manual, for minimized, expanded, heads up notifs
 adb shell settings put global show_new_notif_dismiss [0|1]
 1) disable flag - undershelf unchanged
 2) enable flag - undershelf gone
Change-Id: Id00b76e729f87e44aaede8a033740a51b182559d