If someone is interested, there is Kotlin server PoC demo for Integrity API: https://github.com/mtrakal/integrity-api-server

Not using backend nonce, not validating Integrity API response on BE, just propagating whole API result back to frontend. So this validations should be done there. But it's I think good starting point, because you just edit Config.kt file, provide service-account.json file and your app can connect to "untrusted" backend to try first integration of it.