Use IPV6_MIN_MTU from NetworkConstants in Ikev2Vpn

Instead of sharing the constant from LinkProperties, use the already
defined constant in the NetworkConstants class.

This allows Ikev2VpnProfile to allow depending on non-public
LinkProperties APIs, as LinkProperties is planned to move to
framework-connectivity.

Bug: 174436414
Test: m
Change-Id: I594bb7e81bc7681799c16eff621a5ffd1b29624c