title: PowerManager.userActivity() does not turn the display on
body: <p>I have a count-down timer activity and want to turn the display on when the timer is done. I tried </p>
  
  <pre><code>PowerManager pm = (PowerManager) getSystemService(Context.POWER_SERVICE);
  pm.userActivity(SystemClock.uptimeMillis(), false);
  </code></pre>
  
  <p>but nothing happens, no error reported, display remains off or dimmed. What I'm doing wrong?</p>