Enhance WspTypeDecoder to decode Content Type Parameters

This patch enables WspTypeDecoder to correctly parse content type
parameters as described in the Wap230 WSP specifications
(wap-230-wsp-20010705-a section 8.4.2.24) which are then passed on
as part of the WAP_PUSH intent notification.

It also recognises all Well Known WSP Content types, and simplifies
their retrieval (i.e. a well known content type will always be
available through the WspTypeDecoder.getValueString() method).

Change-Id: I0eb3f9ac287aa7cb53312777c4be54b1939fa857