[backup] Return keyserver device ID in latest backup info endpoint

Summary:
Address server-side of ENG-10236. Implemented approach 2 from task comments.

Depends on D14466

Test Plan: Using Postman, called the endpoint for user with- and without keyserver. Verified that returned device ID matches my keyserver device ID.

Reviewers: kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D14467