Add first OWNERS files for core networking

Covering
  core/java/android/net/
  core/java/com/android/server/net/
  packages/CaptivePortalLogin/
  services/core/java/com/android/server/
    ConnectivityService.java
    NsdServic.java
    NetworkManagementService.java
  services/core/java/com/android/server/connectivity/
  services/net/
  tests/net/

Android core networking files not covered:
  services/core/jni/com_android_server_connectivity_tethering_OffloadHardwareInterface.cpp
  services/core/proto/ipconnectivity.proto (path unique to aosp)

Bug: 63408147
Test: builds
Change-Id: Ifbb923d5ffdcada83864b5e0eb07911682d66efa