Updating Existing CS APIs for multiple defaults

Updating existing ConnectivityService APIs to support multiple default
network functionality.

Bug: 178729499
Bug: 172347841
Test: atest FrameworksNetTests
atest NetworkStackTests
atest FrameworksNetIntegrationTests
atest NetworkStackIntegrationTests
atest CtsNetTestCasesLatestSdk

Change-Id: Ic41fdc402a26809efda71f484c259ffd7a52e63b