This also removes pref overrides from methods like LocaleService::IsLocaleRTL or
IntlService.getLocaleInfo, because it doesn't really make sense to override the
result of checking an arbitrary locale, the relevant use case is overriding the
result for the current app locale.

Removal of the intl.uidirection pref completely will be done in a separate bug.

Differential Revision: https://phabricator.services.mozilla.com/D96235