Split replaceAccount from addAccountToCache

Summary:
We were accidentally overwriting first account in the model with
currently logged in user after polling AccountsService

BUG: 336994

Test Plan: check if kcmshell5 user_manager lists mutliple users on cold boot

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, ngraham, rdieter, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D12837