[Mojo Video Capture] Hook up video capture service behind a feature flag

services/video_capture/service_manifest.json

 {
   "name": "video_capture",
   "display_name": "Video Capture",
   "interface_provider_specs": {
     "service_manager:connector": {
       "provides": {
-        "production": [ "*" ],
+        "capture": [ "video_capture::mojom::DeviceFactoryProvider" ],
         "tests": [ "*" ]
       },
       "requires": {
         "service_manager": [ "service_manager:all_users" ]
       }
     }
   }
 }