This will require some symbol exporting, but the benefit is that have
better control of what external test cases can do in terms of creating
more testing specific contexts.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>
Rather than handle UpClient in both MetaBackend (to reset the idletime
when the lid is opened), and in MetaMonitorManager and
MetaMonitorConfigManager (to turn the screen under the lid on/off
depending on its status), move the ability to get the lid status from
UPower or mock it in one place, in MetaBackend.
