notif: Add vm:entry-point pragma to _onBackgroundMessage

This fixes part of #528, following the suggestion n-92 made here:
  #342 (comment)

Even with this change, notifications still don't yet work on Android
in release builds.  But after this change, when the app wasn't in
the foreground, they get farther than before.  Specifically they get
just as far as they get, with or without this change, when the app
was in the foreground.  Details here:
  #528 (comment)

Author: gnprice

lib/notifications.dart

@@ -174,6 +174,12 @@ class NotificationService {
     _onRemoteMessage(message);
   }
 
+  // This pragma `vm:entry-point` is needed in release mode, when this method
+  // is needed at all (i.e. on Android):
+  //   https://firebase.google.com/docs/cloud-messaging/flutter/receive#background_messages
+  //   https://github.com/firebase/flutterfire/issues/9446#issuecomment-1240554285
+  //   https://github.com/zulip/zulip-flutter/issues/528#issuecomment-1960646800
+  @pragma('vm:entry-point')
   static Future<void> _onBackgroundMessage(FirebaseRemoteMessage message) async {
     // This callback will run in a separate isolate from the rest of the app.
     // See docs: