Use setConfigSettings function instead

Author: Cynthia Jiang

remote_config/src/ios/remote_config_ios.mm

@@ -353,8 +353,10 @@ static int64_t FutureCompleteWithError(NSError *error, ReferenceCountedFutureImp
                                   ::firebase::internal::kMillisecondsPerSecond);
   newConfigSettings.fetchTimeout = static_cast<NSTimeInterval>(
       settings.fetch_timeout_in_milliseconds / ::firebase::internal::kMillisecondsPerSecond);
-  impl().configSettings = newConfigSettings;
-  future_impl_.Complete(handle, kFutureStatusSuccess);
+  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+    [impl() setConfigSettings:newConfigSettings];
+    future_impl_.Complete(handle, kFutureStatusSuccess);
+  });
   return MakeFuture<void>(&future_impl_, handle);
 }