[keyserver] Move accepting policies on siwe registration

Summary:
issue: ENG-8047
We want to use the new way, introduced in D11780: accept the policies inside processSuccessfulLogin by setting the shouldMarkPoliciesAsAcceptedAfterCookieCreation flag

Test Plan:
Tested that when a new user registeres, they don't see the policies modal - the policies were correctly accepted
Tested that if I only remove the call to viewerAcknowledgmentUpdater from processSIWEAccountCreation, but don't set the flag, then the new user does see the policies modal.
Tested that if I set policies as not accepeter for a user and login, they see the modal

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D11918