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.
Move finding, opening and managment of the KMS file descriptor to
MetaMonitorManagerKms. This means that the monitor manager creation can
now fail, both if more than one GPU with connectors is discovered, or
if finding or opening the primary GPU fails.
https://bugzilla.gnome.org/show_bug.cgi?id=785381
