title: Add System Sound to iOS remote notification in UNNotificationServiceExtension
body: <p>I am successfully modifying my remote notification payload using the UINotificationServiceExtension.</p>
  
  <p>I want to change the alert sound based on the user's choice here.  In order to do that I need to assign a UINotificationSound object to the bestAttemptContent:</p>
  
  <pre><code>bestAttemptContent.sound = UNNotificationSound(named: UNNotificationSoundName(rawValue: "alertTone.caf"))
  </code></pre>
  
  <p>This will work for tones which I have created and imported into my project.  One named alertTone.caf in the above example.  </p>
  
  <p>However, I want to use the system alert tones instead of my own.  I know I can set the default tone but I want access to other custom tones.  Is there anyway at all to do this?</p>