A couple of problems with the plugin

[pablomargareto]

The option of "extraPaths" in the environment-rules.json is not really optional:
copyExtraFolders() { this.rules.extraPaths.forEach((filePath) => { this.copyFiles(filePath); }); }
should be changed to something like:
copyExtraFolders() { if (this.rules.extraPaths) { this.rules.extraPaths.forEach((filePath) => { this.copyFiles(filePath); }); } }

Another fail is that is looking for the google-services.*.json on the wrong directory (App_Resources/Android/src/main/res).

You can workaround the two problems with this extraPaths option:
"extraPaths": [ "App_Resources/Android" ],