gtkapplication-dbus: Initialise screensaver-active property

When GtkApplication starts listening to the screensaver's D-Bus
status, the screensaver-active property is not initialised and
applications making use of the property are out of sync until the
first state change.  Any application starting when the screensaver is
active will think it's inactive.

To fix this, we set the property when we first start monitoring the
screensaver.