hs-proxy,hs-clientmanager: Handle correctly the shutdown of apps

The fake subscribe mechanism failed to account for the client context,
which is bound to the afb_req_t of the client itself, and only
dealing with the client list.  This effectively means we can't
really register (a/an automated) callback function to remove the
appid from the client list once the application has been legally
terminated/stopped.

This adds a check to verify, for the showWindow verb, if the application
is still found to be running, and return the appropriate value in case
that is not case. This should determine to start the application and fix
the issue.

Bug-AGL: SPEC-3796

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ia828e1ec374bf3bed21c52814721074c01f16691