How to save RoundedBitmapDrawable

Question

I want to change a shape of image from square to round. And after that I am planning to save this bitmap. But i faced with problem I can't cast RoundedBitmapDrawable to Bitmap? Or may be exist another way for saving RoundedBitmapDrawable instance to SD card.

 void setImage(){
    mBitmap = BitmapFactory.decodeResource(getResources(),R.drawable.user);
    mImageView.setImageDrawable(createNewBitmap(mBitmap));
    mBitmap = ((BitmapDrawable) mImageView.getDrawable()).getBitmap();
    //after that i am planning save bitmap
}

Drawable createNewBitmap(Bitmap bitmap){

RoundedBitmapDrawable roundedBitmapDrawable = RoundedBitmapDrawableFactory.create(getResources(),bitmap);
    roundedBitmapDrawable.setCornerRadius(bitmapRadius);
    roundedBitmapDrawable.setAntiAlias(true);
    return roundedBitmapDrawable;
}