Notif redesign: Show small icon for headless system apps

The implementation to determine if an app iss a headless system app is
the same as the one currently in Notification.java (that's currently
used by a prototype and will be removed).

Bug: 371174789
Test: tested manually for now, unit tests to be added when caching is
implemented
Flag: android.app.notifications_redesign_app_icons

Change-Id: Id93b040f05b6ecea5a41f96a3a66179ba7f58513