Refactor to InterfaceParams utility class

NetworkInterface throws Exceptions every time you look at it askance.
Try to make something we instantiate fully, once, and pass it around.

Partial MacAddress-ification as well, for good measure.

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
    - basic IPv6 tethering works
Bug: 32163131
Bug: 62476366
Change-Id: I16c145fddb4e76387370338d792a438eed886d7f