Reduce advertised ipv6 mtu by 16 to fit ethernet header

This is a temporary hack to workaround the inability of current
kernel's ebpf bpf_skb_change_mode() function to prefix a 14-byte
ethernet header on to a packet without going over the upstream
(source, rawip) interface's mtu *before* we bpf_redirect() to
the downstream (destination, ethernet) interface.

Test: build, atest, atest TetheringTests
Bug: 149816401
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I646148ebfd978a2489c0cd065e4b671b01150add