trust-stored-skeleton does not follow NameOwnerChanged for the Agent

[dobey]

If the service providing the core.trust.dbus.Agent interface for the trust-stored-skeleton process goes away, the skeleton no longer reports accurately whether an app is allowed or denied access to the service, and always reports denied. We are hitting this problem with the new pay-service while trying to integrate trust-store support, as it does not run all the time, to avoid unnecessary memory usage and wakeups when the service is not needed.

Ideally, the trust-stored-skeleton would also not need to run all the time, and would only be activated when necessary (when system-settings needs it, or when an app is performing an action which requires querying for access). At the very least though, I think the skeleton needs to properly handle NameOwnerChanged on the bus, for the dbus.Agent interface.