Use public API of NetworkAgent in TestNetworkService

- Create NetworkAgent by using the public API of NetworkAgent.
- Remove the usage of NetworkInfo.

Bug: 172183305
Test: Build pass.
Test: atest FrameworksNetTests
Test: atest CtsNetTestCasesLatestSdk
Test: atest NetworkStackIntegrationTests
Test: atest TetheringIntegrationTests
Change-Id: Icba59295861201c221587caa9df02275c4087fb8