Update stashed handle region to only include the handle

Before it included the whole width/height of the view, rather than just the outline where the handle is drawn.

Test: Swipe up from a light app on dark wallpaper, ensure handle changes color as soon as the white app background is past halfway through the handle
Bug: 193938970
Change-Id: Ic8b83f60976f05b8ff56590422bd9b2fa6140e76