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
Change-Id: I84078b2241214b3b993ccaf2e590406018df00ff