Fix the takeoff position of a flung windowshade.

We were carefully tracking the vertical offset between the view
origin and the user's incident touch, but not actually
including it when performing a fling. (This might have
worked at one point but been later buried under some
refactoring rubble.)

Bug: 5210198
Change-Id: I97ae883491a5dedf1b48683441096fe9938d118f