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

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

Depends on D5208

Test Plan: verified that wallet login with a new device ID 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/D5209