Apple’s Siri, the voice assistant for iPhone 4S, was touted as iPhone 4S exclusive since it needs the actual processing energy previous iPhones didn’t have. That fantasy was broke when 
designers revealed that the majority of the Siri processing is server aspect, and a few even managed in order to port this to old iPhones.
But based on the opinion of Applidium, Siri utilizes only minimum resources and may easily end up being ported to Android or every other device. One thing which prohibits this it’s a unique signal that recognizes each iPhone with Apple’s servers, and there’s no way of replicating the actual code —at least now. It’s not really unlikely to assume some cyber-terrorist to break the signal and produce new rules for Android along with other devices.