Text selection: Fix word boundaries for languages without spaces

WordIterator's getEnd/getBeginning methods does not support the needs
for word boundary detection in text selection. Consider the words
AABB (where AA and BB are each words). If getEnd is given the offset
on the boundary between AA and BB, it would return that offset since
it is the end of AA. For text selection we always want the "next end"
if available.

This CL adds two methods to word iterator that return the next end
and previous beginning.

This CL also alters the code in Start/EndHandle to use the x value
and current / prev line positions to determine if the user is
expanding or contracting the selection.

Bug: 21305292
Change-Id: I6e7a83e53e245d71e43d78f1957f844f2ed1cdfb