Create notification channel interstitial shelf

From NotificationGuts, tapping "Turn off notifications" will now present
a half-shelf allowing the user to directly block up to 4 channels (the
one given from the notification guts + up to 3 other channels from
that app) or the app itself.

Test: visual (for now)
Bug: 130307442
Fixes: 131432719
Change-Id: I7e82928dfd56b9e25e5bef02607eede55b11d9e3