Avoid breaking activity startup by dbus timeout on sigle instance - Fixes #4773

This problem only happen in activities configured with the option -s
in the activity.info "exec" field, and the objective is use a single process,
even when starting more than one instance. If the process do not reply,
start a new process, instead of show a error.

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>