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

原来，当你从角落开始滑动(特别是edge设为底边，从左下角开始)的时候，activity会可以被拖到与设定的方向垂直的另一个方向。。这个patch将修复此bug