Spell checher's language synced with keyboard.

Bug 5379440. The spell check is now using the IME's language to
do the spell checking. Changing the input language triggers a
new spell check of the entire text.

Optimizations: ArrowKeyMovementMethod re-uses the TextView's
wordIterator, already set to the correct language.

One wordIterator shared by all SpellParsers in SpellChecker.
Cannot re-use TextView's because of concurrency issues.

With the current implementation, one has to type a new character
to see the new spell checking take place.

Change-Id: I0e460c0a6777548f89d03d6b68f3deea6606c17f