Prototype replacing small icons with app icons.

This would replace the small icon in the notification shelf, AOD and
status bar, as well as the notification itself.

As this is just a simple prototype, it's not particularly optimal since
we just want to see what this will look like before investing time in a
proper solution.

Bug: 335211019
Test: manual; no automated tests yet as this is just a prototype
Flag: ACONFIG android.app.notifications_use_app_icon DEVELOPMENT
Change-Id: I0ae3070339efa4558cf97fad3a20b9fe69641cd8