If the context is recognized as a type supported by this creator,
then return an instance of an ADFContext. All parameters are from the
environment of the request. Any parameter (except the context) can be null.
context - The context instance of the ADFContext needed. Never null.
pc - The remainder of the parameters in this creation context.
Never null, but the individual items may be null;
Returns:
null if the context parameter isn't recognized, or an ADFContext.
It may be the instance that was setAsCurrent on the thread when this
method was called.