Text selection: update touch delta when moving towards handles

When the text selection drag handles jump to the end / start of a
word, the delta between the current touch position and where the
handle jumps to is tracked. Previously this value was not updated
if the handle jumped and then the finger moved towards the handle.

This CL adjusts the logic to update the delta in this situation.

Bug: 21131483
Change-Id: I331f53191103e1a3fc08e34b5df5cd6cbc177af6