[compass_app] Mark classes used for namespacing as abstract final (#2540)

As they are just used for namespacing static properties, these classes
shouldn't be extended, implemented, or instantiated.

compass_app/app/lib/config/assets.dart

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-class Assets {
+abstract final class Assets {
   static const activities = 'assets/activities.json';
   static const destinations = 'assets/destinations.json';
 }