title: Android: Draw on bitmap with finger in transparent color
body: <p>I have an ImageView with a bitmap. What I want: When the user draws with the finger on the bitmap this part should become transparent(set Pixel alpha value to 0) that you can see the views under the imageview.
  Can I work with an ImageView or should I implement a custom view?</p>
  
  <p>How can I realize this? (just roughly)</p>