Expose netId to SystemApi

Tethering will be build as unblundled APP. It can't use @hide
API anymore. Expose netId for tethering because it need to set
proxy dns server with netId.

Bug: 144758139
Test: build
Change-Id: Ifefa791940874617bb479f5c23488d14be87f45e

api/system-current.txt

@@ -4175,6 +4175,7 @@ package android.net {
   public class Network implements android.os.Parcelable {
     ctor public Network(@NonNull android.net.Network);
     method @NonNull public android.net.Network getPrivateDnsBypassingCopy();
+    field public final int netId;
   }
 
   public final class NetworkCapabilities implements android.os.Parcelable {

core/java/android/net/Network.java

@@ -63,7 +63,7 @@ public class Network implements Parcelable {
     /**
      * @hide
      */
-    @UnsupportedAppUsage
+    @SystemApi
     public final int netId;
 
     // Objects used to perform per-network operations such as getSocketFactory