Fix issue where handle is in front of word rather than end of word

Previous getWordEnd and getWordStart functions would return a boundary
rather than a word end / start boundary in some cases. This behavior is
most evident when moving between short lines -- the handle would go to
the next boundary rather than the word end on the next line (and the
start handle would go to the word end rather than the word start on
the prev line).

This CL ensures that word or punctuation boundaries are returned and moves
most of the punctuation boundary logic into WordIterator since it makes
a bit more sense there.

Bug: 21030788
Change-Id: I96c6aff7f2c213aa3c4f66ac87ca913ca16fd347