Insertion cursor: add some slop for moving between lines

Applying same method to insertion cursor as the text selection drag
handles to improve moving along lines.

Basically adds some slop above / below the current line and only
allows you to change lines if you move outside of that.

Bug: 21306433
Change-Id: I6c7f3a496fbd1ea66936832f96325736cea872aa
1 file changed