gsd-power: Let gsd actually change brightness

The first commit ensures we supply values [0.0, 1.0] instead of either 0.0 or 1.0. The 2nd makes the brightness logic trigger at all. It is not ideal as we don't have the current backlight brightness available anymore (maybe we need to extend the API so that g-s-d can query the current values?)

I've stumbled on this while adding support for the new API to phosh but checked that gnome-shell 49.0 (as currently packaged in Debian) is affected as well.

This only makes any difference when

org.gnome.settings-daemon.plugins.power ambient-enabled is set to true