Adding support for multiple "default" networks

Updating ConnectivityService foundational support for multiple default
networks by extending mDefaultRequest and mDefaultNetworkNai from single
instances to a HashMap to support more than a single default network.

Bug: 176027457
Bug: 172347841
Test: atest FrameworksNetTests
atest NetworkStackTests
atest FrameworksNetIntegrationTests
atest NetworkStackIntegrationTests
atest CtsNetTestCasesLatestSdk

Change-Id: I2f5fa96c2d21ab883499e82afa74e73f3ca606cd