title: No custom sound with Android Firebase Notification
body: <p>I am using Firebase push notifications in my Android App. I can send correctly notification with custom icon, but I have not managed to play my custom sound. I always get the default sound of my device. </p>
  
  <pre><code>{
      "registration_ids": "myToken",
      "notification": {
          "body": "my body",
          "title": "my title",
          "icon": "ic_notification",
          "sound": "mysound.mp3" // I tried "mysound", "mysound.wav"...
  
      },
      "priority": "high"
  
  }
  </code></pre>
  
  <p>The custom sound is located in /res/raw</p>
  
  <p>I have been able to play my custom sound with <code>onMessageReceived</code> and Firebase data message <strong>but not with Firebase notification message</strong>.</p>
  
  <p>My android device is Xiaomi Mi A1 and Oreo 8.1., also tried with Xiaomi Mi A2 with same result.</p>
  
  <p>I tried with php and curl, with node.js... always same problem, I get my default sound.</p>
  
  <p><strong>UPDATE</strong></p>
  
  <p>With this code for node.js does not work either:    </p>
  
  <pre><code>var registrationToken = 'xxxxxx';
  
  var message = {
  
      notification: {
        title: 'my title',
        body: 'my body',
      },
      android: {
        ttl: 3600 * 1000,
        notification: {
          color: '#ff0000',
          sound: 'mysound.mp3'
        }
      }, 
      token: registrationToken
  
  };
  </code></pre>