notif: Move NotificationOpenService init at the start of NotificationService init

NotificationOpenService.instance.start already handles all the
platforms itself, by doing nothing on all except iOS, so move it's
initialization out of the platform-specific switch here.

Author: rajveermalviya

lib/notifications/receive.dart

@@ -53,6 +53,8 @@ class NotificationService {
   ValueNotifier<String?> token = ValueNotifier(null);
 
   Future<void> start() async {
+    await NotificationOpenService.instance.start();
+
     switch (defaultTargetPlatform) {
       case TargetPlatform.android:
         await ZulipBinding.instance.firebaseInitializeApp(
@@ -79,8 +81,6 @@ class NotificationService {
         await _getFcmToken();
 
       case TargetPlatform.iOS: // TODO(#324): defer requesting notif permission
-        await NotificationOpenService.instance.start();
-
         await ZulipBinding.instance.firebaseInitializeApp(
           options: kFirebaseOptionsIos);