DERBY-6186: Encapsulation improvements

- Use DataDescriptorGenerator.newTriggerDescriptor() to build descriptor
  in SYSTRIGGERSRowFactory instead of calling constructor directly

- Make TriggerDescriptor's constructor package private

- Make UserDescriptor's constructor package private

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1469240 13f79535-47bb-0310-9956-ffa450edef68