Bootstrap MAC address retrieval on Oreo using a peer

[aarmea]

Currently, on Oreo and up users need to manually copy the MAC address from Android settings into Noise. Because peers will have the other's address once a connection is established, it can send it back to the MAC-unaware device. For this to happen, only one device needs to be broadcasting a beacon.

In the Thali project, this is known as Bro Mode.