Merge IInputMethodManagerInvoker and IInputMethodManagerGlobalInvoker

This CL mechanically merges

  IInputMethodManagerInvoker

into

  IInputMethodManagerGlobalInvoker

without changing any observable semantics.  This allows us to remove

  InputMethodManager#mServiceInvoker

completely.

This refactoring makes it much easier for us to see whether a method
in InputMethodManger is implemented only with static methods or not.

Bug: 234882948
Test: presubmit
Change-Id: I310a33f27b876419b6a3058202dcbd663b541079