code: public static synchronized void setDefaultChannelId(String id) {
          try {
              if (_setDefaultChannelId(id)) {
                  save();
              }
          } catch (Exception ex) {
              Log.e(WonderPush.TAG, "Unexpected error while setting default channel id to " + id, ex);
          }
      }
docs: Set the default channel id.
  
  <p>
  This function does not enforce the existence of the given channel until a notification
  is to be posted to the default channel.
  This way you are free to call this function either before or after creating the given
  channel, either using this class or directly using Android O APIs.
  </p>
  
  @param id The identifier of the default channel.