Bug 971521: [System2] Instantiable NfcUtils.

This change moves classes in the nfc_utils.js file into a modular/export format.

Class NfcUtils now can be instantiated. NfcBuffer was always instanced. NDEF is still init()'ed since it is intended for mostly const usage only.

(NfcManager change is coming too. May have some conflicts: Bug 971520)