Text selection: add some slop for moving between lines

It's pretty easy to accidentally move to a different line when
selecting text. This CL makes an adjustment so that if the user
is on a line, they must move above the top of it + slop to move
up a line, and they must move below the bottom of it + slop to move
down a line.

This CL also makes a small modification to check if the layout is
null and return early which simplifies some of the following code.

Bug: 21306433
Change-Id: I4a854e644faafaf1e159201ae1f2838862f16b25