Add test coverage for registerSystemDefaultNetworkCallback. am: 1e5cdd3187

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: If5381eec59318dd01b6c19b13165dbacb6c86af2
tree: f9a6e0e16499e1d16c3e35dda488b3b6d37187e2
  1. tests/
  2. Tethering/
  3. .gitignore
  4. OWNERS
  5. PREUPLOAD.cfg
  6. TEST_MAPPING