Add support for themed app icons in notifs

This is a simple prototype so that we can see how these look. It
currently has the following limitations:
1) does not take into account the user's preference for themed icons
2) icons of existing notifications do not get updated on theme change

Bug: 388377895
Flag: android.app.notifications_redesign_themed_app_icons
Test: visual test
Change-Id: I8cdeb40cfb5e992169c48cb98bc013f1e1a90cd9