akonadi: improve debug message in state() method

Summary:
"already starting" sounded like we're trying to start it again,
but that's a copy/paste from the start() method. Here it can also
mean that we're in the process of shutting down.

Test Plan: Ran one of the isolated tests, output makes more sense now

Reviewers: dvratil, mlaurent

Reviewed By: dvratil, mlaurent

Subscribers: mlaurent, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D25455