title: Flutter: local notifications custom sound doesn't work
body: <p>I'm trying to play my own custom sound for my notifications. But on my android emulator, it just plays the default sound and on my own device, it doesn't even make a sound. 
  I tried <a href="https://stackoverflow.com/a/59931642/11979688">this</a> but it didn't help me.</p>
  
  <pre class="lang-dart prettyprint-override"><code>
  FlutterLocalNotificationsPlugin _notifications = FlutterLocalNotificationsPlugin();
  
  var initializationSettingsAndroid = AndroidInitializationSettings('app_icon');
  var initializationSettingsIOS = IOSInitializationSettings();
  var initializationSettings = InitializationSettings(initializationSettingsAndroid, initializationSettingsIOS);
  await _notifications.initialize(initializationSettings);
  var androidPlatformChannelSpecifics = AndroidNotificationDetails('test_channel', 'test', '', playSound: true, sound: RawResourceAndroidNotificationSound('my_sound'));
  var iOSPlatformChannelSpecifics = IOSNotificationDetails();
  var platformChannelSpecifics = NotificationDetails(androidPlatformChannelSpecifics, iOSPlatformChannelSpecifics);
  await _notifications.show(0, 'test', 'this is a test', platformChannelSpecifics);
  
  </code></pre>
  
  <p>I don't get any errors. My file is in android/app/src/main/res/raw/my_sound.mp3</p>