hs-proxy: Keep track of clientCtx and client when starting application

hs-proxy: Keep track of clientCtx and client when starting application

The subscribe verb was used in the past to keep track of applications,
which was done in background with the help of event handlers. As
that no longer happens we keep track of the client and its clientCtx
when starting directly from the homescreen binding, faking the subscribe
logic.

As the afb_req_t object is no longer in scope when the async callback
handler returns, we do it before doing the call to af-main, and assume
that we'll succeed.

We use the async callback handler to  verify if the application was
indeed started but any application crashing afterwards will require an
additional check performed, as it will found in the hash table of
clients.

Bug-AGL: SPEC-3524

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I0e417dc5f2953947cd27ca551c0d2a2af5c57e6d