Add a systemapi constructor for NetworkAgent

Bug: 138306002
Bug: 139268426
Test: atest FrameworksNetTests FrameworksWifiTests FrameworksTelephonyTests
      make doc-comment-check-docs
Change-Id: I288ea32fac07a9a486e2ea451a2c9b098446a74c
Merged-In: I288ea32fac07a9a486e2ea451a2c9b098446a74c
3 files changed