BlackboardAttribute API change

Make two changes consistent with deprecation of context field begun in earlier releases:

1. Make constructor with context arg used by SleuthkitCase have package access instead of protected access. There is no reason to allow the more lax access for the convenience of BlackboardAttribute subclasses - we don't want this class to be a base class and probably should also mark it as final.
2. Deprecate constructor that takes a byte array value and a context string and provide an overload that takes a byte array value but no context string.