title: Android heads-up notification not showing
body: <p>I am trying to make heads-up notification work. Notification is created, but it's not displayed at the top of the app.
  Here's the code responsible for building a notification:</p>
  
  <pre><code>Notification notification = new NotificationCompat.Builder(context)
                                  .setSmallIcon(android.R.drawable.arrow_up_float)
                                  .setContentTitle("Check running time - click!")
                                  .setContentText(String.valueOf(elapsedTime))
                                  .setContentIntent(pendingIntent)
                                  .setDefaults(Notification.DEFAULT_ALL)
                                  .setPriority(Notification.PRIORITY_HIGH)
                                  .setVibrate(new long[0])
                                  .build();
  </code></pre>
  
  <p>The device that I'm trying to run the app is API 21. I've seen many threads, but no solution given works for me. </p>