Maximize VCN MTU when using IPv6

This commit maximizes VCN MTU by considering IP version. Specifically,
VcnGateWayConnection stores/updates the IkeSessionConnectionInfo passed
out from the IKE library during IKE setup and migration. By looking into
the addresses stored in the IkeSessionConnectionInfo, VCN is able to
know the IP family of the VCN tunnels.

Bug: 222331693
Test: atest FrameworksVcnTests(new tests), CtsVcnTestCases
Change-Id: I9a2839d7827f1d6451546ec222129f286e94ae36