WFD use of HDCP is now optional.

if the sink supports it we'll use HDCP (and fail if necessary), if it doesn't
we won't. If an HDCP session is established we'll tell our observer that
the connection is secure, otherwise we don't.

Change-Id: I7cbef384f2cf0a6ac65801c581eea227b9ef4c46
related-to-bug: 7368436