Smart Home sample using Android Things, Firebase, and Cloud functions

Build

Create a new project on the Firebase console and note the Project ID somewhere (you'll need it soon)
Add an Android app to this project, with the following package name: com.nilhcem.smarthome.androidthings
Download the google-services.json file and save it to the app directory
Create a new Firebase Realtime Database, importing the following json file

{
  "fan" : {
    "on" : false
  },
  "lights" : {
    "on" : true,
    "spectrumRGB" : 16510692
  }
}

{
  "rules": {
    ".read": true,
    ".write": true
  }
}

Go to Firebase's Project Settings > Service Accounts, and click on the "Generate New Private Key" button. Save the generated file in servers/smart-home-provider-functions/serviceAccountKey.json
Open the servers/smart-home-provider-functions/index.js file use your Firebase Project ID to modify the databaseURL variable
Deploy the Android app to your Android Things device. At that time, modifying the database values from the Firebase console should have an effect on the Android Things device.

Enable the Cloud Functions API and install the Google Cloud SDK (quickstart guide)
Deploy the ha (home automation) function with the following command

cd servers/smart-home-provider-functions
gcloud beta functions deploy ha --stage-bucket staging.<PROJECT ID>.appspot.com --trigger-http

This script will deploy the function to Google Cloud and give you the endpoint address. Keep the address somewhere, you'll need it (something like https://us-central1-<PROJECT ID>.cloudfunctions.net/ha).

Note: if you want to test the functions locally, use the Cloud Functions Emulator

functions-emulator start
cd servers/smart-home-provider-functions
npm install
functions-emulator deploy ha --trigger-http
functions-emulator stop

A fake OAuth2 mock server is provided so you can test Actions on Google easily. Do not use this one on production.
To build and start it, run the following commands:

cd servers/fake-oauth-server-nodejs
npm install
npm start

At that time, the server will run on localhost:3000. Download ngrok to expose it to the Internet

ngrok http 3000

The fake OAuth2 server is now exposed to the url. Copy the url (similar to https://<NGROK_ID>.ngrok.io). You'll need it later

Update the servers/action.json file and specify the Google Cloud Functions endpoint to the url variable
Create an Actions on Google project on the Actions Console using the Actions SDK
When told to update the app via the gactions command, run the following to use our action.json file

cd servers
gactions update --action_package action.json --project <PROJECT_ID>

Start the Google Home app on your phone
Go to the devices settings > Home control > Add device and select the [test] device
Login (user: rick, password: oldman)
You will now be able to see the fan and the lights devices
You can now control those devices from the Google assistant