minor bugfix for ios push notifications config (#1614)

In the "apns" key of the message sent to FCM, "headers" should not be
nested under "payload", it should instead be at the same level. See
Firebase docs for the `FirebaseAdmin.Messaging.ApnsConfig` class
[here](https://firebase.google.com/docs/reference/admin/dotnet/class/firebase-admin/messaging/apns-config#class_firebase_admin_1_1_messaging_1_1_apns_config_1a7a9858f0b30ef7c437205ca01d6e62f6)

engine/apps/mobile_app/tasks.py

@@ -216,11 +216,11 @@ def _get_fcm_message(alert_group, user, registration_id, critical):
                         "interruption-level": "critical" if critical else "time-sensitive",
                     },
                 ),
-                headers={
-                    # From the docs
-                    # https://firebase.google.com/docs/cloud-messaging/concept-options#setting-the-priority-of-a-message
-                    "apns-priority": "10",
-                },
             ),
+            headers={
+                # From the docs
+                # https://firebase.google.com/docs/cloud-messaging/concept-options#setting-the-priority-of-a-message
+                "apns-priority": "10",
+            },
         ),
     )