systemd service which owns display should indicate as much

Change-Id: Ieef44c12ae0acf70905b4c0930a4c2f108e8b0a9

resources/artriculate@.service

@@ -16,3 +16,4 @@ Restart=on-failure
 
 [Install]
 WantedBy=graphical.target
+Alias=display-manager.service