Potential thread concurrency issue with WordIterators.

parse() is called from the UI thread when text is changed, and in
that case it is safe to re-use the TextView's WordIterator.

It can also be called from the SpellCheckSession thread though
onGetSuggestions: use a local WordIterator in that case.

Change-Id: I596bff15cc8f997b9dc49598724450ac5c582101