Add @hide SpellCheckerSubtype#getLocaleObject().

This is a mechanical refactoring with no behavior change.

With this CL, InputMethodSubtype and SpellCheckerSubtype have the same
getLocaleObject() hidden API, which makes it easy to share the logic in
subsequent CLs.

Bug: 11736916
Bug: 20696126
Bug: 22858221
Change-Id: I39dc0c310158ad23ba6c987efce07deaf30ce693