ENYO-398: Eliminate "jump" after dragging during animated scroll

If you are using enyo.Scroller with enyo.TouchScrollStrategy (or a
subkind like enyo.TranslateScrollStrategy), and you drag to scroll
while an animated scroll is already in progress, then the scroller
will jump to the wrong position when you stop dragging.

This is because enyo.ScrollMath keeps track of the final position
it's scrolling to and doesn't properly reset it when dragging.

To fix, we reset this.endX and this.endY in startDrag().

Enyo-DCO-1.1-Signed-Off-By: Gray Norton (gray.norton@lge.com)