Make sure notif icon has a bg

If we're showing the small icon like before, use the same coloring (the
"inverted" style where the icon itself is the color of the notification,
and the background is either provided by the developer or just based on
the theme). If we're showing the app icon, don't add additional padding
and make sure we have a white background.

Ideally, we'd want to use what launcher does to scale the icons properly
and add the background, but this is an initial prototype.

Bug: 335211019
Test: tested some apps manually
Flag: android.app.notifications_use_app_icon_in_row DEVELOPMENT
Change-Id: Ic681cb343b9416931eccbe92d7a2568540b49d9f