Notif redesign: Use launcher app icons for notification rows

This is just the simplest implementation for fetching the icon properly.
AppIconProvider will have a cache, and we'll also need to not show app
icons for certain notifications, but that will be in follow-up CLs. I
also didn't add tests yet since the actual implementation in
AppIconProvider will change soon.

One KI I noticed is this doesn't seem to work for group headers yet.

Bug: 371174789
Test: manually post notifications and see that they have the right icon
Flag: android.app.notifications_redesign_app_icons
Change-Id: Iad9f68877c6c3bc356505d30036ba4dc1166847e