Use references in agent_dvr strings.json (#40999)

homeassistant/components/agent_dvr/strings.json

@@ -11,7 +11,7 @@
       }
     },
     "abort": {
-      "already_configured": "Device is already configured"
+      "already_configured": "[%key:common::config_flow::abort::already_configured_device%]"
     },
     "error": {
       "already_in_progress": "Config flow for device is already in progress.",