refactor(core): call DeviceModelInfo::config_key model_key

It is the model's key — {extended_model_id}{model_id}, the depot manifest's modelId — and since the identity-driven device list it has not keyed per-device configuration; DeviceIdentity::config_key does. Two of its three callers already put the result in a field or variable named model_key. The doc says which key is which.