Improve monitoring of seat state

The new seat state monitoring in 0.5 is very nice and mostly works very well in allowing bluetooth devices to be used by whoever is currently logged in on the console across fast user switches.

I say mostly because it goes wrong as soon as one of the users has a remote login to the machine, because that session will always appear to be active and will stop them relinquishing control of bluetooth when they are no longer active on the console.

This MR changes the logic so that if the user is reported as active then we check their sessions to verify that at least one session that is associated with a seat is active and downgrade the status to online if not.