title: Delete notification channel without id
body: <p>My app has a bug where it sends different push notification and up until now every notification got its own channel ID, so the user has got for every new notification a new channel which he can then enable/disable by toggle. This bug is now fixed but I want to delete the old unnecassary channels programmatically and I found the way do it like this: </p>
  
  <pre><code>// The id of the channel.
  val notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
  val id: String = "my_channel_01"
  notificationManager.deleteNotificationChannel(id)
  </code></pre>
  
  <p>But since every channel got created by a random generated ID and this ID has been  discarded I can't delete the channel by his ID. </p>
  
  <p>Is there a way to list all channel IDs a user got or any other way how I can delete old channels without knowing the ID</p>