[TL02]Remove hidden API usage of NetworkAgent

The connection service will become the mainline module.
Remove the hidden API usage of NetworkAgent.

Bug: 170598012
CTS-Coverage-Bug: 170598012
Test: atest FrameworksNetTests FrameworksTelephonyTests
      atest FrameworksWifiTests
Change-Id: I4e4040ae7f94bdf479c7df9ec2ffabafbe06331c