ViewDragHelper: allow only one edge to be touched by one finger

this fixes the bug when drag a corner of the view, without this you're going to see the view being draged to a different direction from which is set by class SwipeBackLayout
Merge pull request #17 from PeterCxy/master

ViewDragHelper: allow only one edge to be touched by one finger
Merge branch 'master' of https://github.com/Issacw0ng/SwipeBackLayout