SEPOLICY: Move WFD service context definitions to private

 -- As per treble guidelines, wfd type and service context
    definitions have to be in private/public folders, as
    having it in common folder will make it get listed
    as default labelled service.
 -- Moving wfd type and service_context definitions to
    private folder.

Change-Id: I803678facae4cbad7d6707ee70914daefcfa3fd2