Fix difficulty grabbing handles for small selections

The gravity for the handles pushes them to the outside of the touch target
square, causing the touch target to overlap for both of them when close
together. This CL swaps the gravity so that the touch target does not
overlap when the handles are close.

Bug: 20065564
Change-Id: I4d14e7076ac779c5f4a0a603d6348ba6dc7c7b4c