fix potentially bug on accessing global variable on instance of AtCore

Summary:
I am not sure if the static here was intentional, but I bet it was not.

The static member makes it virtually impossible for two instances of AtCore to co-exist. Even though the architecture of the library itself seems to not have been though to allow multiple instances, now doing it would be a design flaw, as a library should never make assumptions about how applications use them.

Reviewers: rizzitello, patrickelectric, laysrodrigues, tcanabrava

Reviewed By: rizzitello, patrickelectric

Subscribers: laysrodrigues, patrickelectric, leandrosantiago, rizzitello

Differential Revision: https://phabricator.kde.org/D14891