google-services.json file for each flavor in android studio

I have different productFlavors specified in my build.gradle file

dev {
        applicationId ""
        versionCode 83
        versionName "2.2.1"
    }
staging {
        applicationId ""
        versionCode 119
        versionName "2.8.1"
    }

both flavors have different project ids etc for google services. i have placed respective google-services.json file in assests folder in each flavor like this

src>dev>assests>google-services.json

and

src>staging>assests>google-services.json

google-services json must be present on root level on compilation.

what should i have to specify in my build.gradle file so that respective file or content should be copied to root folder's file when i build a particular flavor.