Make most of NetworkAgent @SystemApi.

Bug: 138306002
Bug: 139268426
Test: atest FrameworksNetTests FrameworksWifiTests FrameworksTelephonyTests
Test: atest android.net.cts.ConnectivityManagerTest
Change-Id: I856f01f02c35c2f6a886d8b98039e5dfbc48148b
Merged-In: I856f01f02c35c2f6a886d8b98039e5dfbc48148b
(cherry picked from commit 631f33a68f45a0194688bbd111f8a112751e1bd7)
2 files changed