1/N The road to Material NEXT (bouncer and shade)

Styling bouncer and shade so they have solid backgrounds and read colors
from themes instead of named resources.

Scrim now fetches colorBackgroundFloating, and views get their background
from colorBackground.
Switching from light to dark theme will make bouncer colors update without
re-inflating any views.

Bug: 173561906
Bug: 173561901
Test: atest ScrimControllerTest
Test: atest KeyguardSecurityContainerControllerTest
Test: manual, switching to dark theme annd showing notif guts
Change-Id: I372e8b688d7bbddd698381accd074b7655788cd0