Introduce IInputMethodClientInvoker

This helps us consolidate binder-related boilerplate from

  InputMethodManagerService

to a separate utility class.

Having this wrapper is also an important step towards removing

  InputMethodManagerService#executeOrSendMessage()

by emulating async IPC semantics when

  IInputMethodClient

is running in the system_server process.

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

Bug: 234882948
Test: presubmit
Change-Id: I9087eafa3f25b2754ca88ecd920aa3079bd0c7b9