Support adding multiple tethering upstreams.

Currently, when the first tethering interface pair is added, we
add MASQUERADE rules for the upstream and IPv6 counting rules.
Continue to add IPv6 counting rules when the first interface pair
is added, but change the code to add MASQUERADE rules every time
a new upstream is added.

This will allow us to support adding, say, v4-rmnet_data0 as an
upstream once we're already using rmnet_data0 as an upstream.

Bug: 38218697
Bug: 64382985
Bug: 64976379
Bug: 64995262
Bug: 64380515
Test: netd_unit_test passes, manual: IPv4 or 464xlat tethering works
(cherry-pick of aosp I84078b2241214b3b993ccaf2e590406018df00ff)
Merged-In: I7747c81c62cc9ce26768849873a76a7e7478ea66
Merged-In: If80af6b3225316c47f11ab39e7544cfb7e095d37

Change-Id: I984ccf335fbc0c630fb21e0edc752a3ebbcd7706