Clear notification undershelf background

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 background is transparent

Change-Id: Icf2a0d0116e8a2c7263b370060676135f8830587