server: clear forwarding rules when tethering is disabled

Since disabling NAT needs tethered iface's address info,
but sometimes the iface goes down before disabling NAT, this fix
just caches the iface's address.

Change-Id: Ie3f14093e8673c34821a3c86d1470c7dda3f3a42
CRs-fixed: 672354
Merge "server: clear forwarding rules when tethering is disabled" into kitkat