title: PowerManager newWakeLock.acquire() not working as expected
body: <p>I want my app to keep the CPU running but turn the screen off to minimise power wastage.</p>
  
  <p>Previous posts on this subject suggest the following approach:</p>
  
  <pre><code>    mPm = (PowerManager) getSystemService(Context.POWER_SERVICE);
      mWakeLock =  mPm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, Constants.WAKE_LOCK_TAG);
  
      mWakeLock.acquire();    //keep CPU running, turn screen off
  </code></pre>
  
  <p>The manifest contains:</p>
  
  <pre><code>    &lt;uses-permission android:name="android.permission.WAKE_LOCK" /&gt;
  </code></pre>
  
  <p>In my innocence I expected that on acquiring the lock the screen would go off immediately but nothing happened.
  The earlier posts I have read never seem to overcome this problem, did anyone succeed?  Is there anything else I must do?</p>