hs-proxy: Avoid creating a client context

The subscribe mechanism requires that the client itself performs the
subscribe (it assumes a unique, distinct afb_req_t type in order to
create the client context) and attempting to handle one,
automatically would not really work, so only use it (the fake sub mechanism)
the keep track of application w/ the help of the client_list.

Bug-AGL: SPEC-3796

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I783614e1db83280878b77b786e4f80fccd1a74a1