code: @Implementation(minSdk = Build.VERSION_CODES.O)
    protected void deleteNotificationChannelGroup(String channelGroupId) {
      if (getNotificationChannelGroup(channelGroupId) != null) {
        // Deleting a channel group also deleted all associated channels. See
        // https://developer.android.com/reference/android/app/NotificationManager.html#deleteNotificationChannelGroup%28java.lang.String%29
        // for more info.
        for (/* NotificationChannel */ Object channel : getNotificationChannels()) {
          String groupId = ReflectionHelpers.callInstanceMethod(channel, "getGroup");
          if (channelGroupId.equals(groupId)) {
            String channelId = ReflectionHelpers.callInstanceMethod(channel, "getId");
            deleteNotificationChannel(channelId);
          }
        }
        notificationChannelGroups.remove(channelGroupId);
      }
    }
docs: Delete a notification channel group and all notification channels associated with the group.
  This method will not notify any NotificationListenerService of resulting changes to
  notification channel groups nor to notification channels.