ENYO-593: Scroll thumb doesn't properly hide in some cases

We recently made a change to the thumb-showing and -hiding logic
in TouchScrollStrategy, calling showThumbs() from the
onScrollMathScroll handler. We neglected to call delayHideThumbs(),
however, which resulted in the thumb not properly disappearing in
some edge cases.

Enyo-DCO-1.1-Signed-Off-By: Gray Norton (gray.norton@lge.com)