Address API review feedback

Address API review feedback to:
 - Rename NetworkAgent#setTeardownDelayMs to
   NetworkAgent#setTeardownDelayMillis
 - Use getters instead of fields in VpnTransportInfo
 - Rename registerDefaultNetworkCallbackAsUid to
   registerDefaultNetworkCallbackForUid in ConnectiivityManager

Bug: 183972850
Bug: 185246410
Fix: 184735863
Test: make update-api
Test: atest FrameworksNetTests
Test: atest CtsNetTestCasesLatestSdk
Test: atest FrameworksMockingServicesTests
Change-Id: I5e8c4bed8bda40d507afa894c359b5e24ee5d868