Android's clat is for reaching the internet, which in general
never has an ipv4 L3 mtu higher than 1500.

(We could probably hit this on a jumboframe enabled IPv6-only
wifi network [ http://b/292057969#comment18

 ] where RA claims
MTU of 9K [9170])

Bug: 292057969
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I7a32455571458b8a3f1121ad980d54323b2ef06b