Fix bug in EdgeEffect with glow

Fixes: 182499389

With the glow edge effect, using the drag gesture to reduce
the edge glow could cause the glow to flip and repeat. This
was caused by the update() modifying the determined distance
during the pull when it wasn't animating.

We also force the alpha to be 0 if the pull distance is 0.

Test: manual testing
Change-Id: Iacc1df0fcb92e3380523187badf7ea62d7d19d2f