Fix: line slop shouldn't always increase / decrease by one line

Touch slop is from the bottom (or top) of the line + line height / 2.
It only makes sense to apply touch slop if the user is within a line
from the previous line. Additionally, not doing this can cause some
undesirable behavior if the user moves very quickly and the selection
catches up with a weird line by line selection increase, potentially
even having the selection be stalled until a next move event.

This CL alters the logic so that if the user isn't within one line
of the previous selection, it'll just use whatever line the user is
currently on.

Bug: 22385003
Change-Id: I4f37988893868e5e2b7925314fe824c3da9c1b97