Simplify IMMS#updateInputMethodsFromSettingsLocked() a bit

This CL simplifies

  InputMethodManagerService#updateInputMethodsFromSettingsLocked()

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: 345519864
Test: presubmit
Flag: EXEMPT refactor
Change-Id: Ic8d803624b90e04a52ddf72381634df4c628a103