It actually doesn't need to be so elaborate. Even a video camera with the lens cap on generates more than enough entropy. Your phone can mix together predictable but unique variations - time of day, free memory, CPU serial number, battery level - with less predictable physical sensory - light level, gyroscope, barometer, last touch points, nearby MAC addresses - to create far more on-board randomness than anyone realistically needs.

That said, the whole Cloudflare lava lamp thing is very cool and also gets people talking.