[native] Broadcast device list update and send backup key stub

Summary:
Primary device should broadcast the device list update to all peers. However, we don't have peer syncing yet, so as a stub, it only broadcasts it to all devices from its device list.
Also, stubbed message containing backup data key, which will be needed for backup restore on secondary device.

Depends on D11296

Test Plan: Added a few devices to the device list - made sure that after successful QR auth flow, all devices receive the device list update. Also, secondary device displays a console log with stub backup key.

Reviewers: kamil, michal

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11301