Move net-test-utils to its own directory am: 0fb2de46f0 am: 3d9ec0a4b8 am: 3423b195bc

Original change: https://android-review.googlesource.com/c/platform/frameworks/libs/net/+/1654919

Change-Id: Ie5bcf5db72d54eac10e853f1d8e94474eed5eadd