Fix Tethering of multiple downstream ifaces.

We weren't notifying the second tethered iface of it's upstream iface name so its traffic
was not getting routed correctly.
We also weren't clearing out our connected iface name when we untethered, so the route rules
weren't getting flushed when the last tether came down.

Change-Id: I34e5d672ea882c89c8f582d69d6dc421cb52d4b4