FieldValues skipping but not updating last used at (#27655)

* FieldValues skipping but not updating last used at

Related to #26863

When fieldvalues synching was being skipped, the
`get-or-create-full-field-values!` call was erroneously forgetting to
update the `last_used_at` time; the time was only being updated if new
field values came in since the last sync.

* Add test for inactive field values

* Only update last_used_at when there's an existing fieldvalues