fix(firebase_messaging): Ensure onMessage callback is consistently called on iOS platform. (#8202)

Author: russellwheatley

packages/firebase_messaging/firebase_messaging/ios/Classes/FLTFirebaseMessagingPlugin.m

@@ -303,11 +303,7 @@ - (void)userNotificationCenter:(UNUserNotificationCenter *)center
     NSDictionary *notificationDict =
         [FLTFirebaseMessagingPlugin NSDictionaryFromUNNotification:notification];
 
-    // Don't send an event if contentAvailable is true - application:didReceiveRemoteNotification
-    // will send the event for us, we don't want to duplicate them.
-    if (!notificationDict[@"contentAvailable"]) {
-      [_channel invokeMethod:@"Messaging#onMessage" arguments:notificationDict];
-    }
+    [_channel invokeMethod:@"Messaging#onMessage" arguments:notificationDict];
   }
 
   // Forward on to any other delegates amd allow them to control presentation behavior.
@@ -490,7 +486,6 @@ - (BOOL)application:(UIApplication *)application
                         }
                       }];
     } else {
-      [_channel invokeMethod:@"Messaging#onMessage" arguments:notificationDict];
       completionHandler(UIBackgroundFetchResultNoData);
     }