title: Camera.setPreviewCallback( PreviewCallback ) not working
body: <p>I'm trying to get camera preview data, but I don't want to show the preview. Unfortunatelly </p>
  
  <pre><code>setPreviewCallback( PreviewCallback )
  </code></pre>
  
  <p>doesn't work unless you call</p>
  
  <pre><code>setPreviewDisplay(SurfaceHolder)
  </code></pre>
  
  <p>Is it possible to pass this in any way - start callbacks without setting the preview display, or any way of hiding the display?</p>