[web] don't generate or send one time keys on login

Summary:
use getExistingDeviceKeyUpload to get the identity keys and prekeys, but not one time keys. note that we do want to generate/send one time keys in secondary device auth so i've left that method alone.

Depends on D11362

Test Plan: logged in from web and confirmed that no new one time keys were added to ddb

Reviewers: ashoat, kamil, marcin, michal

Reviewed By: ashoat, michal

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11363