title: How to make hit area for android button ? like flash hit area
body: <p>I want to know how to make 'hit area' for an android button.</p>
  
  <p>I have a button with background image. This image is non-uniform shape. I want to make a 'hit area' and a 'non-hit area' -- the area with shape is the 'hit area', transparent area is 'non-hit area'. </p>
  
  <p>The background image is composed of a transparent area and a non-transparent area.</p>