hammerhead: allow interfaces to get v6 address when tethering is enabled
hammerhead: allow interfaces to get v6 address when tethering is enabled

From kernel/Documentation/networking/ip-sysctl.txt
accept_ra - INTEGER
    Accept Router Advertisements; autoconfigure using them.

    It also determines whether or not to transmit Router
    Solicitations. If and only if the functional setting is to
    accept Router Advertisements, Router Solicitations will be
    transmitted.

    Possible values are:
        0 Do not accept Router Advertisements.
        1 Accept Router Advertisements if forwarding is disabled.
        2 Overrule forwarding behaviour. Accept Router Advertisements
          even if forwarding is enabled.

    Functional default: enabled if local forwarding is disabled.
                        disabled if local forwarding is enabled.

Change-Id: I7d91c05ab378deca1d4b153e70a52c34e2cb47ab

maruos/android_device_lge_hammerhead,maruos/android_device_lge_hammerhead,maruos/android_device_lge_hammerhead,maruos/android_device_lge_hammerhead