9to5Mac was able to confirm that the code is found in iOS 7.1 and iOS 7.1.1 files but not in iOS 7.0.6.
The file is a supporting property-list resource for Assistant (Apple’s internal name for Siri), containing information about the feature. The UIDeviceFamily array declares what platforms are compatible with the feature. As shown by Apple’s documentation, '1' and '2' represent the iPhone/iPod touch and iPad families. The '3' represents the Apple TV family.
It's believed that Apple is working on a new generation Apple TV which will be released soon. The device is rumored to feature new content, gaming capabilities, AirPort Express functionality, and even a cable box feature. We can now add Siri to the list of possible improvements!