Properly enable EDM4U for Unity 2021.2. Fixed googlesamples#477

Update `.meta` for editor libraries used in `.tgz` so that Unity
2021.2+ can properly load them.

Author: chkuang-g

export_unity_package_config.json

@@ -22,11 +22,15 @@
           ],
           "override_metadata_upm": {
             "PluginImporter": {
-              "platformData": {
-                "Editor": {
-                  "enabled": 1
+              "platformData": [ {
+                  "first" : {
+                      "Editor": "Editor"
+                  },
+                  "second": {
+                      "enabled": 1
+                  }
                 }
-              }
+              ]
             }
           }
         },