fix(messaging, iOS): revert onMessage event handler commit which causes another bug (#6878)

Author: helenaford

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

@@ -286,8 +286,11 @@ - (void)userNotificationCenter:(UNUserNotificationCenter *)center
     NSDictionary *notificationDict =
         [FLTFirebaseMessagingPlugin NSDictionaryFromUNNotification:notification];
 
-    // Send an `onMessage` event
-    [_channel invokeMethod:@"Messaging#onMessage" arguments:notificationDict];
+    // 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];
+    }
   }
 
   // Forward on to any other delegates amd allow them to control presentation behavior.
@@ -398,10 +401,7 @@ - (void)application:(NSApplication *)application
         [FLTFirebaseMessagingPlugin remoteMessageUserInfoToDict:userInfo];
 
     if ([NSApplication sharedApplication].isActive) {
-      // Only handle data-only messages from FCM.
-      if (userInfo[@"aps"][@"alert"] == nil) {
-        [_channel invokeMethod:@"Messaging#onMessage" arguments:notificationDict];
-      }
+      [_channel invokeMethod:@"Messaging#onMessage" arguments:notificationDict];
     } else {
       [_channel invokeMethod:@"Messaging#onBackgroundMessage" arguments:notificationDict];
     }