Reduce member fileds in InputMethodMenuController

With this CL, InputMethodMenuController no longer needs to keep
maintaining the following members.

 * InputMethodSettings
 * InputMethodSubtypeSwitchingController

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

Bug: 309868254
Bug: 309837937
Test: presubmit
Change-Id: Ib63e796d289cc310a3200c15dc50b1f675f9ba0d