Adapt to new KAccounts plugin system
Summary: KAccounts was ported from QPluginLoader to KPluginLoader in D27633. Adapt the KAccounts plugin to stay compatible
Test Plan: Added a google account. Got a notification that my drive can now be used
Reviewers: KIO GDrive, elvisangelaccio
Reviewed By: KIO GDrive, elvisangelaccio
Differential Revision: https://phabricator.kde.org/D27656