DO NOT MERGE Call mutate on notification shade drawable before editing.

            // shade is visible to the user. When the notification shade is completely open then

            // alpha value will be 1.

            float alpha = (float) height / mNotificationView.getHeight();

            Drawable background = mNotificationView.getBackground();

            Drawable background = mNotificationView.getBackground().mutate();

            background.setAlpha((int) (alpha * 255));

            mNotificationView.setBackground(background);

        }

    }