[identity] Return use_new_flow in V1 login RPCs

Summary:
Address part of ENG-9697.

Depends on D14073

Test Plan: Used primary device logout to sign the device list, then tried V1 login again, the use_new_flow was returned.

Reviewers: kamil, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D14074