title: what is notification channel id?notifications not work in api 27
body: <p>I updated my project api target to 27 and all notifications were disabled.<br>
  What's the difference between notification in api 26 and 27?</p>
  
  <pre><code>        Notification notif = new NotificationCompat.Builder(this)
                  .setContentTitle(getString(R.string.app_name))
                  .setContentText(message)
                  .setSmallIcon(R.mipmap.ic_launcher)
                  .setContentIntent(pIntent)
                  .setSound(alarmSound)
                  .setAutoCancel(true).build();
          notif.ledARGB = 0xFFff0000;
          notif.flags = Notification.FLAG_SHOW_LIGHTS;
          notif.ledOnMS = 100;
          notif.ledOffMS = 100;
  
          NotificationManager notificationCompatManager =
                  (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
          notificationCompatManager.notify(0, notif);
  </code></pre>