[services][identity] update user's public keys in ddb during PAKE login

Summary:
If a user logs in, we should update the users table to reflect the potentially new deviceID-userPublicKey mapping.

We should refactor these service methods to not loop. Tracked here.

Depends on D5200

Test Plan: verified that PAKE login on a new device updates the devices map in dynamodb to reflect the new public key

Reviewers: tomek, jon

Reviewed By: tomek, jon

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D5208