Cleanup IME subtype switching controller and tests

This adds annotations for the nullability of parameters, values and
return values of methods in the IME subtype switching controller, as
well as simplifying the existing logic with an equivalent one.
Additionally fixes the testImeSubtypeListItem to check the
isSystemLanguage of all items, as well as add the missing item_en_uk.
Lastly, replaces the non-inclusive language from the test class.

Test: atest InputMethodSubtypeSwitchingControllerTest
Bug: 347693610
Change-Id: I6292fd34f016d7a1635994a364d91eb071c5d5b5