We currently store build-time configuration flags in AppConstants.swift. We should either add the Activity Stream feature here or create a new Experiments.swift file to contain user-controllable flags to store our experimental features.
Activity stream is enable now in nightly and beta. I've added a configuration flag in AppConstants.swift. The first phase of AS is pretty similar to top sites. I dont think users will need an opt in flag. master fdc3684c7c071328eb40bda08a8740dbec6fff41