Add requirements for IPv6 implementation.

This is intended to:

1. Formalize the requirements that we've been testing in CTS for
   many years now.
2. Prevent some observed bad device behaviours that can severely
   impact user experience on IPv6-capable networks, which are now
   quite common.
3. Clarify the treatment of IPv6 in doze mode, which is
   non-obvious enough that even we only fixed it close to final M
   release.

Bug: 24919272
Change-Id: Ia739b242529e831c63e53e55e490df243e831c80
1 file changed