Adjust TTL for ipv6 tethering

If upstream is cellular, set the TTL in Router Advertisements to
"network-set TTL - 1" for carrier requirement. For other non-cellular
upstream, set TTL as "network-set TTL + 1" to preventing arbitrary
distinction between tethered and untethered traffic.

Bug: 154776299
Test: atest TetheringTests

Change-Id: I7f2696a642f96c6aafb5613b980bf5bcdd08bbda