Don't break IPv6 connectivity when in doze mode.

Working IPv6 connectivity relies on the kernel being able to
receive certain ICMPv6 packets (router advertisements, neighbour
solicitations, neighbour advertisements) at all times. Allow
these packets when in doze mode.

This is not necessary for IPv4 because in IPv4 these functions
use ARP, which is invisible to iptables.

Bug: 23158230
Change-Id: I29ed77561db9688486cf58cd14ac3bce7fce4b40