ScreenLockerWatcher was written whislt overthinking DBus.

An interface does not have to have the same lifespan as the service the
other end. DBus connections works in terms of established "match rules",
these can be set up before a service is created the other side and they
remain if a service closes. The only part that needs doing is re-fetch
any cached properties.

There's also need to check the intial service names at startup. We can
just let our call to GetActive fail. It's far faster.

This class was getting convoluted in both ifdefs and Qt6 porting all to
solve something that isn't real.

There should be no behavioural differences.