Make IMMS#getCurrentInputMethodSubtypeLocked() multi-user aware

In order to remove the remaining dependencies on

  InputMethodManagerService#mCurrentUserId,

this CL makes

  InputMethodManagerService#getCurrentInputMethodSubtypeLocked()

multi-user aware.

There must be no observable behavior change.

Bug: 350386877
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I83844835ad710a23f46d3b95f1d4014ffff273ed