Plugin java: fix notification time conversion to cdtime_t.

Fix conversion of time in notifications to ``cdtime_t``.

To reproduce the problem:

Read notitications:

    import org.collectd.api.Collectd;
    import org.collectd.api.Notification;
    import org.collectd.api.CollectdNotificationInterface;
     
    public class TestNotify implements CollectdNotificationInterface
    {
   	public TestNotify ()
   	{
   		Collectd.registerNotification("TestNotify", this);
   	}
       
   	public  int notification (Notification n)
   	{
   		System.out.println(n.toString());
   		return 0;
        }
    }

Generate notifications:

    import org.collectd.api.Collectd;
    import org.collectd.api.Notification;
    import java.util.Date;
    import org.collectd.api.CollectdReadInterface;
     
    public class TestDispatchNotification implements CollectdReadInterface
    {
       	public TestDispatchNotification ()
    	{
   		Collectd.registerRead("TestNotify", this);
   	}
       
   	public int read()
   	{
   		Notification n = new Notification();
   
   		n.setTime(new Date().getTime());
   		n.setSeverity(Notification.WARNING);
   		n.setHost("localhost");
   		n.setPlugin("java");
   		n.setPluginInstance("TestDispatchNotification");
   		n.setType("counter");
   		n.setMessage("TestDispatchNotification");
   
   		Collectd.dispatchNotification(n);
   
   		return 0;
   	}
    }

The cmd ``PUTNOTIF`` has the same problem, fixed in https://github.com/collectd/collectd/pull/644