[identity] Update login date for existing devices

Summary:
A supplement for D10398 - when existing device calls login RPC again, the access token is minted again. We don't need to update the whole device list, but just update the login date so the primary device can be determined correctly.

Depends on D10398

Test Plan: Called login_password_user a few times and used awslocal dynamodb to verify that login date is updated correctly with each call.

Reviewers: varun, michal

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10399