This project was done in a bit of a rush. I was already working on a Bluetooth Low Energy peripheral device using the Raspberry Pi when I heard a co-worker was leaving. I had the peripheral working as a kind of "remote command line" - I can send the NodeJS BLE peripheral short-lived terminal commands and send the result back to the phone via a notification.
So I added a new characteristic: A motor controller that the Android device can send a message in order to run motors at a specific speed. Then it was just about building the bot.