Respect sentence boundary when doing spell check.

Introduces SentenceIterator into android.widget.SpellChecker and
uses it to detect the start/end of spell check range instead of
using a fixed length.

Fix: 179125303
Fix: 179124679

Test: atest CtsInputMethodTestCases:SpellCheckerTest

Change-Id: If6dd044e24dcaa58381949c221a5f9af57a4104f