[web] Retrieve username on secondary device login

Summary:
Addresses ENG-7215 on web.
Now we have username in RCPs response, so we can simply pass it to identityAuthResult, replacing the empty string placeholder.

Depends on D12134.

Test Plan: Console.log the identityAuthResult in the uploadKeysForRegisteredDeviceAndLogIn() and confirmed the username was there during QR login.

Reviewers: inka, varun, will

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12135