MDL-58486 user_prefs: always update $USER cache when applicable

MDL-58486 user_prefs: always update $USER cache when applicable

set_user_preference and unset_user_preference let you pass the user to
set preferences for as an id. Previously, if you passed $USER->id there,
those methods did not update the Cache in $USER, which was surprising,
and not easy to debug.

Now, we always update the preference cache in $USER if the preference is
being changed for the current user.