Make IMMS#onActionLocaleChanged() multi-user aware

With this CL, Intent#ACTION_LOCALE_CHANGED will be fully propagated
into any visible background users when

  InputMethodManagerService#mConcurrentMultiUserModeEnabled

is set. There should be no behavior change otherwise.

Bug: 350386877
Test: presubmit
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: Ifc7aebeef5f3354703fa1de668996e7f4691b900