[native] Don't bother with keyserver auth in BackgroundIdentityLoginHandler

Summary:
We don't need to do it here, since we're already auth'd with the authoritative keyserver.

We just need to perform identity auth.

This mitigates ENG-8725 and ENG-8726, and addresses ENG-8779.

Test Plan: Confirm that I'm able to go from a release build running with usingCSAT=false to one running with usingCSAT=true. Exact instructions here. Make sure that the two "mitigated" issues are no longer present, and that the upgrade goes without issue (except for ENG-8778, which is a separate issue)

Reviewers: varun, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12678