Drag to select text: 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 applies the changes to the drag accelerator.

This CL also makes an adjustment to not apply the finger offset
to the drag accelerator until the user has moved lines, this was
causing an issue where it would immediately include the above line
when initiating the selection.

Bug: 21306433
Bug: 21303943
Change-Id: Id240136493f62524fe6445d8bd65479084fe8126