korgac: only try to start Akonadi if it has been configured

Summary:
korgac is autostarted on login. We don't want it to start up Akonadi
though if the user has not set up Akonadi yet. In that case (i.e. when
akonadiserverrc does not exist yet), korgac will wait for the server
to be started and only then continues to set up ETM and other Akonadi
stuff. If Akonadi has been configured previously, then korgac will
just continue to create ETM and thus start Akonadi if necessary right
after login.

Reviewers: kde-pim

Subscribers: lbeltrame, winterz

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D2050