Add test coverage for registerSystemDefaultNetworkCallback. am: 1e5cdd3187 am: 1f40096f3e

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/1570840

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I3ece3d8ce25be1d024376539ba4c9c17e9d15d7b
tree: a78099cc196263ccb9690e110ffcf9deed614967
  1. tests/
  2. Tethering/
  3. .gitignore
  4. OWNERS
  5. PREUPLOAD.cfg
  6. TEST_MAPPING