Setup interfaces for IPv6 tethering

Including:
    - set the interface for router mode (accept_ra = 0)
    - reset the interface for client mode (accept_ra = 1)
    - InterfaceController::setAcceptIPv6Ra()
    - InterfaceController::setAcceptIPv6Dad()
    - make InterfaceController static
    - refactor for more modern C++ usage here and there
    - sporadic style guide fixes

Bug: 9580643
Change-Id: Ia557c8770e18c58b12ad16d982c63b6ebd525516