Add IMMS#getCurrentImeUserId() to reduce inter-class dependencies

In order to clarify the data dependency between *Controller classes
and InputMethodManagerService, this CL introduces

 InputMethodManagerService#getCurrentImeUserId()

to reduce the direct references to InputMethodUtils from controller
classes.

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

Bug: 309868254
Test: presubmit
Change-Id: I70b64921f676513be400129b8a79f35a28776d60