brightness-manager: Add auto-brightness handling

I had proposed a MR in g-s-d to fix brightness as it is currently non-functional in GNOME 49 (not only for phosh). As that seems stalled let's do it here. This initial version uses a simple display independent bucket approach for different ambient levels to determine the backlight brightness to avoid drastic brightness changes. I have some improvements pending for #803 and to take the panels curve into account but that can be done as follow ups. Having automatic brightness back again will already be a bonus. The algorithm is pluggable so we can experiment with different versions relatively easily (e.g. adding EMA). This needs more testing but putting it here in case someone wants to try it out.