Don't prompt for Offline Storage unless disk activity is enabled

Despite setting browser.cache.offline.enable to false, we still get prompted for allowing offline storage when sites try to use it. Canonical example: http://html5demos.com/offlineapp

The prompt appears to be governed by browser.offline-apps.notify, which we don't set.

I am pretty sure that this prompt runs through the permissions service, which we make memory only (legacy/trac#2950 (closed)), but we should double check that through direct test.