Introduce IInputMethodManagerInvoker

This helps us consolidate binder-related boilerplate code from

  InputMethodManager.java

to a separate utility class.

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

Bug: 234882948
Test: presubmit
Change-Id: Ifdb676cf0e37165eb4992f649ac0b694590afc7c