Restore the notification background's hovered/pressed tints

As far as I know, NotificationBackgroundView always has the "custom
background" set to R.drawable.notification_material_bg, which is a
layer-list with two layers:

The bottom layer is the real notification background color, which (at
the moment) defaults to materialColorSurfaceContainerHigh, and should be
replaced with the tint color if the notification is colorized.

The top layer is a selector that blends in a state-dependent amount of
the notification foreground color, which (at the moment) defaults to
materialColorOnSurface, and should be replaced with the calculated
foreground color if the notification is colorized. This is what gives
the notification background's default, hovered, and pressed states a
distinct appearance.

Right now, neither of those things are happening. At first, we were
tinting the entire LayerDrawable with the tint color, but *only when not
hovered or pressed*, so the notification would de-colorize when hovered
or pressed.

I "fixed" that in ag/25397894 by tinting it in all three states, but
then the default, hovered, and pressed states became indistinguishable.

This change specifically updates only the *bottom* layer with a
colorized notification's background color, so the default, hovered, and
pressed states are all colorized *and* distinguishable.

This change does *not* update the *top* layer with a colorized
notification's *foreground* color, which would result in more
distinguishable states for some color situations, but would also require
much more plumbing of colors.

Bug: 277285099
Bug: 310318993
Test: manual: observe default/hovered/pressed states of colorized notif
Flag: NA
Change-Id: Ie2d256fa73155647742293f8fc5db33a2053db8b