Add a simple test for Ipv6Utils. am: 98ff7450ab am: 50f4a073e0 am: 651dc51178

Original change: https://android-review.googlesource.com/c/platform/frameworks/libs/net/+/1636001

Change-Id: I654c5ea1f26a0e0c039f9be3ced8f09c81651062