Support tethering IPv6 toward the first requested downstream.

We no longer tether toward wifi only, but rather implement 64share toward
the current, oldest (first requested) active downstream.

Additionally, explicitly ban IPv6 tethering toward Bluetooth until
PAN support is figured out.

Bug: 30298058
Change-Id: Ib705b69eae767ec50131d09d26168d9e041e4bc0