Add test coverage for registerSystemDefaultNetworkCallback. am: 1e5cdd3187 am: 1f40096f3e am: c49cba3526

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I09f367980efec9969e8b8b711757fd2909189e24
tree: 0546e97773d9b15716e492429685eb29f6c01d56
  1. tests/
  2. Tethering/
  3. .gitignore
  4. OWNERS
  5. PREUPLOAD.cfg
  6. TEST_MAPPING