Code review comment for lp://staging/~mir-team/qtmir/iteration-0-of-miral-PersistDisplayConfig

> We already have a policy in qmirserver

Added a first pass at incorporating this.

Also plumbed in use of the DisplayConfigurationObserver::base_configuration_updated() coming in Mir-0.26.

RAOF has picked up supplying the EDID - with that we'll be in a position to follow up by tracking hardware changes and persisting the stuff we want.