SYSTRIGGERSRowFactory should use DataDescriptorGenerator to build descriptor

SYSTRIGGERSRowFactory.buildDescriptor() creates a descriptor by calling TriggerDescriptor's constructor directly. It already has a DataDescriptorGenerator instance, so it should instead call the newTriggerDescriptor(). This would allow TriggerDescriptor's constructor to become package protected, which improves encapsulation.