[lib] broadcast updates when becoming aware of new device existence

Summary:
This handles two cases:

1. Login - this logic will be executed on the device logged in using legacy Identity auth notifying other peers.
2. Whenever becoming aware of another user - ping him to fetch our newest device list.

Note: This code will be removed in the future after fully launching secondary device auth.

Depends on D12379

Test Plan:

1. Run local TB, and Identity, and make all clients connect to it.
2. usingCommServicesAccessToken = true
3. Login on multiple devices (2 devices of user A, 1 device of user B but A is aware of B).
4. Login on a new device of user B.
5. All A's and B's devices should receive a device list update.

Reviewers: bartek, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12380