Simplify IMMS#setInputMethodLocked() a bit

This CL simplifies

  InputMethodManagerService#setInputMethodLocked()

a bit by explicitly having a local instance "userId" instead of using

  settings.getUserId()

as a synonym of it.

This is a mechanical refactoring CL, which should have no behavior
change.

Bug: 305849394
Test: presubmit
Flag: EXEMPT refactor
Change-Id: Iba73348d92ce818ecc93a88ca63baf13834a3251