Allow text selection handles to scroll horizontally

When selecting text and expanding the selection, the handles snap
to the end (or start) of words. The handles don't snap until the
user has moved halfway through the word.

In horizontally scrolling text views, where some of the text is
cut off, the user cannot be halfway through the word, this causes
the selection to get stuck unless the user scrolls the view and then
continues selecting.

This CL does two things:

1) Checks if the user is close to the edge of the view when the
view can scroll horizontally, and places the cursor at next offset
if available.

2) Moves the code to check if handles are crossing into own method
this should be done each time the cursor is placed and avoids the
need to duplicate the check throughout updatePosition code.

Bug: 22657879
Change-Id: Ic14cb0994cd202a897bf6532f3832bb93ed49bfb