Use Activator for FAB URL handler instead of Blueprint

The Blueprint file in fab-osgi bundle for the fab: URL handler specifies a mandatory service reference on the FeaturesService. In a Fabric-managed container, FAB will function fine without this, so it should be made optional instead.

Also, the registration of the FAB URL handler should be done through a BundleActivator instead of through Blueprint, to ensure the handler gets installed without waiting for other stuff (like the shell namespace handler) in the Fabric agent.