Wouldn't this kill battery life? Or are there third-party apps/Android apps that run all the time with little effect on battery life?
Or could it perhaps be done with a dedicated coprocessor? Like the M7/M8 but instead of motion, it's always listening and working out whether you're saying "Hey Siri" or not?
My Samsung galaxy note 3 has this feature. Prettty sure my samsung galaxy note 2 could too. You can have it always listening for the phrase "hi galaxy" to use s-voice or OK google commands. The battery life used by this is very little.