Shopware 5.2 - register cron with plugin

gistfile1.txt

/**
* Valid for SW 5.2 - 5.2.14
* Since 5.2.15 cronjob.xml can be used https://developers.shopware.com/developers-guide/plugin-system/#plugin-cronjob
*/

public static function getSubscribedEvents() {
    return [
    'Shopware_CronJob_ReminderMail' => 'onCronRun'
    ];
}

/**
* install method
* @param InstallContext $context
*/
public function install(InstallContext $context) {
    parent::install($context);
    $this->addCron();
}

/**
* uninstall method
* @param InstallContext $context
* @return boolean
*/
public function uninstall(UninstallContext $context) {
    parent::uninstall($context);
    $this->removeCron();
}

public function addCron()
{
    $connection = $this->container->get('dbal_connection');
    $connection->insert(
        's_crontab',
        [
        'name'             => 'ReminderMail',
        'action'           => 'ReminderMail',
        'next'             => new \DateTime(),
        'start'            => null,
        '`interval`'       => '900',
        'active'           => 1,
        'end'              => new \DateTime(),
        'pluginID'         => null
        ],
        [
        'next' => 'datetime',
        'end'  => 'datetime',
        ]
        );
}

public function removeCron()
{
    $this->container->get('dbal_connection')->executeQuery('DELETE FROM s_crontab WHERE `name` = ?', [
        'ReminderMail'
        ]);
}

public function onCronRun(\Shopware_Components_Cron_CronJob $job) {

}