notif [nfc]: Change _groupKey method to take a realm URL and user ID

`_groupKey` method takes an `FcmMessageWithIdentity` object as an argument. This object is only used to get the realm URL and user ID. So, it makes sense to change the method to take `realmUrl` and `userId` directly. This change simplifies the method's signature and improves code clarity.

Discussion:
  https://chat.zulip.org/#narrow/channel/516-mobile-dev-help/topic/Removing.20notifications.20for.20logged.20out.20account/near/2088813

Author: tomlin7

lib/notifications/display.dart

@@ -1,10 +1,10 @@
 import 'dart:async';
 import 'dart:io';
 
-import 'package:http/http.dart' as http;
 import 'package:collection/collection.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart' hide Notification;
+import 'package:http/http.dart' as http;
 
 import '../api/model/model.dart';
 import '../api/notifications.dart';
@@ -231,7 +231,7 @@ class NotificationDisplayManager {
   static Future<void> _onMessageFcmMessage(MessageFcmMessage data, Map<String, dynamic> dataJson) async {
     assert(debugLog('notif message content: ${data.content}'));
     final zulipLocalizations = GlobalLocalizations.zulipLocalizations;
-    final groupKey = _groupKey(data);
+    final groupKey = _groupKey(data.realmUrl, data.userId);
     final conversationKey = _conversationKey(data, groupKey);
 
     final oldMessagingStyle = await _androidHost
@@ -365,7 +365,7 @@ class NotificationDisplayManager {
     // There may be a lot of messages mentioned here, across a lot of
     // conversations.  But they'll all be for one account, so they'll
     // fall under one notification group.
-    final groupKey = _groupKey(data);
+    final groupKey = _groupKey(data.realmUrl, data.userId);
 
     // Find any conversations we can cancel the notification for.
     // The API doesn't lend itself to removing individual messages as
@@ -445,10 +445,10 @@ class NotificationDisplayManager {
     return '$groupKey|$conversation';
   }
 
-  static String _groupKey(FcmMessageWithIdentity data) {
+  static String _groupKey(Uri realmUrl, int userId) {
     // The realm URL can't contain a `|`, because `|` is not a URL code point:
     //   https://url.spec.whatwg.org/#url-code-points
-    return "${data.realmUrl}|${data.userId}";
+    return "$realmUrl|$userId";
   }
 
   static String _personKey(Uri realmUrl, int userId) => "$realmUrl|$userId";