Move AkStandardDirs to KF5AkonadiPrivate

Akonadi::StandarDirs provides instance-aware access to Akonadi config files
unlike XdgBaseDirs::akonadi*ConfigFile() which always point to the global
files regardless of current instance settings. The plan is to port clients
to use Akonadi::StandardDirs instead and remove the broken functions from
XdgBaseDirs.