kcms/clock: fix date/time save on non-systemd distros
kcm_clock defaults to using timedated to set date and time where available. Timedated is usually non available on non-systemd distros, where kcm_clock uses a "legacy" fallback. The "legacy" fallback has 2 problems:
- after KAbstractConfigModule::setNeedsAuthorization() was removed, an explicit call setHelperId() is needed for the helper to be actually called;
- the values for "newdate" and "olddate" were always the same, so no change was actually applied.