notif test [nfc]: Make messages list returned by getActiveNotificationMessagingStyleByTag non-growable

Doing this better simulates real bindings behaviour.

Author: rajveermalviya

test/model/binding.dart

@@ -621,7 +621,7 @@ class FakeAndroidNotificationHostApi implements AndroidNotificationHostApi {
                   key: message.person.key,
                   name: message.person.name,
                   iconBitmap: null)),
-            ).toList());
+            ).toList(growable: false));
     }
   }