Unify IMMS#onActionLocaleChanged() behaviors among users

This is a follow up CL to my previous CL [1], which made our
Intent#ACTION_LOCALE_CHANGED handling multi-user aware.

One thing that has not been fully addressed is a special case for
background users when IMMS#mConcurrentMultiUserModeEnabled is false.

With this CL we start applying the same logic for background users to
eliminate remaining special casing around IMMS#mCurrentUserId.

No abrupt observable behavior change is expected/intended.

 [1]: Ifc7aebeef5f3354703fa1de668996e7f4691b900
      686cb71d

Bug: 350386877
Flag: build.concurrent_input_methods
Test: presubmit
Change-Id: I6e9666d482cc37c604a5edf9e54cddb51c0f6ec6