Remove white background from masked image in android

Question

I want to crop a bitmap in polygon shape drawn above that bitmap. I'm using masking of bitmap to do this. The operation succeeds well, unfortunately the resulted bitmap after masking has a white background in cropped area,as you can see in the image:

And my mask that is created runtime by dragging rectangle around image and creating a polygon shape on above the image is as below : I want to remove this unwanted white background. Does any one has idea how could I do this. Any clue or help about how can I do is greatly appreciated. Thanks in advance.