Inline IME subtype switching ControllerImpl

This acted mostly as a wrapper which lead to some duplicated code and
documentation, so it can be easily inlined.

The only difference is visible in tests: when updating the controller
with new items, it won't return a new item, but rather mutate the
existing one. Otherwise, callers wouldn't directly access
ControllerImpl, so this remains equivalent.

Flag: EXEMPT REFACTOR
Bug: 347693610
Test: atest InputMethodSubtypeSwitchingControllerTest
Change-Id: Ic0525781ee02c3416c6b27c220759603a5eb9615