applets/brightness: Update applet status on dbus service changes
Watch for the DBus services register and unregister signals to update the service availabilty, so the UI always reflects the current status, in case something changes after initialization, or the services are started later than the applet
Test Plan
With the applet open, start and stop the powerdevil service vía systemctl and check how the status is updated.
BUG: 486067
FIXED-IN: 6.3