Use events constants when generating event subscriber.

[Chi-teck]

Currently the subscribed events are referenced to by hardcoded string literals.

public static function getSubscribedEvents()
{
    return [
        'kernel.request' => 'onKernelRequest',
    ];
}

I think referencing events with constants is more reliable.

public static function getSubscribedEvents()
{
    return [
        KernelEvents::REQUEST => 'onKernelRequest',
    ];
}