Simplify InputMethodSubtypeSwitchingController a bit

This simplifies InputMethodSubtypeSwitchingController a bit by taking
advantage of InputMethodSettings, which is in general a super set of
InputMethodMap and user ID.

This is a mechanical refactoring CL. There must be no semantic
behavior change.

Bug: 305849394
Bug: 346662143
Bug: 347693610
Flag: EXEMPT refactor
Test: atest InputMethodSubtypeSwitchingControllerTest
Change-Id: Ibc1410c6a49401983a5216a1b8973fb95f117e20