Make MockVpn more realistic and easier to use. am: 02e6be7824

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/13425312

Change-Id: I0e1ff975206ed180744c4c7a6f42e00b95589424
tree: d8d61a393a381fe0d1523a70495698a3b0bca66a
  1. core/
  2. services/
  3. tests/