title: how to get the color from GradientDrawable
body: <p>First, I set the green color to be the background of the View mIcon,</p>
  
  <pre><code>View mIcon = findViewById(R.id.xxx);
  
  GradientDrawable gdraw = (GradientDrawable) mContext.getResources().getDrawable(R.drawable.roundbtn_white_normal);
  gdraw.setColor(Color.GREEN);
  mIcon.setBackgroundDrawable(gdraw);
  </code></pre>
  
  <p>Then, I don't know how to get the color from this View's background... there's no getColor() 
  function...</p>