[identity] Forbid registering multiple devices with the same ID

Summary:
Addresses ENG-8383.
We don't want to allow registering multiple users with the same device ID.
However, if given device ID belongs to the same user, we can allow that (details in Linear task).

Depends on D12349

Test Plan: Commtest: called register_user_device() twice with the same device keys. Second call failed due to Status::AlreadyExists

Reviewers: varun, will

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12350