Remove the handle headroom if the challenge is open.

This change also moves to an absolute-distance model of
tracking the panel instead of a delta model; this allows us
to make the decision about the handle size in only one place
(when first detecting the gesture) instead of requiring us
to mix it into the arithmetic everywhere.

Also fixes the drag-too-slowly-and-the-panel-doesn't-move
problem.

Change-Id: I9bdbadd968134ad5ad67eaef9539536fb5cfe1a1
Proto-Id: I0c2f484da89f063995384c263aa9f5f03339ed7c