Fix a bug in sentence detection when input is too long.

When the unchecked length of the current sentence is too long, the
spell checker should check the first MAX_SENTENCE_LGNTH characters of
the unchecked part. In this case, detectSentenceBOundary should return
[textChangesStart, textChangeStart + MAX_SENTENCE_LENGTH)

Fix: 188875278

Test: manual test
Change-Id: I31847aed2d564f7bc1ff43c83adae3bb7c99c0d1