retry: Add @hide SpellCheckerSubtype#getLocaleObject().

This is the 2nd try of I39dc0c310158ad23ba6c987efce07deaf30ce693.

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.

No behavior change is intended.

Bug: 11736916
Bug: 20696126
Bug: 22858221
Change-Id: I51be014c752b736a808e2b0d56e664941a218a2f