power: Don't react to light changes if not at console

As iio-sensor-proxy isn't yet clever enough to only send property
changes signals to clients that have claimed a sensor, disconnect
the properties-changed signal so we don't attempt to make brightness
changes when we're not on the active seat.

See https://github.com/hadess/iio-sensor-proxy/issues/210
See https://bugzilla.gnome.org/show_bug.cgi?id=756539
See https://bugzilla.gnome.org/show_bug.cgi?id=773685
See https://bugzilla.gnome.org/show_bug.cgi?id=764896
See https://bugzilla.redhat.com/show_bug.cgi?id=1322588

https://bugzilla.gnome.org/show_bug.cgi?id=792409