title: React native firebase messaging notification sound not change in background
body: <p>i am using @react-native-firebase/messaging for push notification in my application.
  and for custom notification sound i use @notifee/react-native this.</p>
  <p>my current notification code is :</p>
  <pre><code>import messaging from '@react-native-firebase/messaging';
  import notifee from '@notifee/react-native';
  
  await notifee.createChannel({
        id: 'custom-sound',
        name: 'System Sound',
        sound: 'notification.mp3',
  });
  
  messaging().onMessage((remoteMessage) =&gt; {
        notifee.displayNotification({
          title: remoteMessage.notification.title,
          body: remoteMessage.notification.body,
          android: {
            channelId: 'custom-sound',
          },
        });
  });
  
  
  </code></pre>
  <p>the above code works fine when the application is open on the screen so that my custom notification sound works perfectly.</p>
  <p>but when the app is running in the background then the notification sound is the default (system default)</p>
  <p>so how i can set the custom notification sound in all cases.</p>
  <p><a href="https://i.stack.imgur.com/uWrzI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/uWrzI.png" alt="This my notification payload" /></a></p>