[identity] Add devices to the new list after login and register

Summary:
Updated LoginUser and RegisterUser DDB calls to double-write new device info to both "legacy" and new device list.

Depends on D10220

Test Plan: Used keyserver to first register, then login (after restart). Used awslocal dynamodb scan to inspect both tables - both were updated successfully.

Reviewers: varun, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10221