message: fix: Make StopDeviceCmd use Id, not SystemId
  Probably accidental copypasta, but means that currently, StopDeviceCmd
  can't be sent by a client.
  Fixes
diff: "type": "object",
  "description": "Stops the all actions currently being taken by a device.",
  "properties": {
  - "Id": {"$ref": "#/components/SystemId"},
  + "Id": {"$ref": "#/components/Id"},
  "DeviceIndex": { "$ref" : "#/components/DeviceIndex" }
  },
  "minProperties": 2,
  