Expose netId by adding getter API

Bug: 151156820
Test: m
      atest TetetheringTests
Change-Id: Ieb1483c146aa2f7d8f251157e6e81d71c44ae899
(cherry picked from commit 8831c4b54178753013d568c0b76bbf7487db8122)

api/system-current.txt

@@ -6270,8 +6270,8 @@ package android.net {
 
   public class Network implements android.os.Parcelable {
     ctor public Network(@NonNull android.net.Network);
+    method public int getNetId();
     method @NonNull public android.net.Network getPrivateDnsBypassingCopy();
-    field public final int netId;
   }
 
   public abstract class NetworkAgent {

api/test-current.txt

@@ -1819,6 +1819,7 @@ package android.net {
 
   public class Network implements android.os.Parcelable {
     ctor public Network(@NonNull android.net.Network);
+    method public int getNetId();
     method @NonNull public android.net.Network getPrivateDnsBypassingCopy();
   }
 

core/java/android/net/Network.java

@@ -64,7 +64,7 @@ public class Network implements Parcelable {
      * The unique id of the network.
      * @hide
      */
-    @SystemApi
+    @UnsupportedAppUsage
     public final int netId;
 
     // Objects used to perform per-network operations such as getSocketFactory
@@ -169,6 +169,17 @@ public class Network implements Parcelable {
         return new Network(netId, true);
     }
 
+    /**
+     * Get the unique id of the network.
+     *
+     * @hide
+     */
+    @TestApi
+    @SystemApi
+    public int getNetId() {
+        return netId;
+    }
+
     /**
      * Returns a netid marked with the Private DNS bypass flag.
      *

packages/Tethering/src/com/android/server/connectivity/tethering/Tethering.java

@@ -1458,7 +1458,7 @@ public class Tethering {
             } else {
                 dnsServers = mConfig.defaultIPv4DNS;
             }
-            final int netId = (network != null) ? network.netId : NETID_UNSET;
+            final int netId = (network != null) ? network.getNetId() : NETID_UNSET;
             try {
                 mNetd.tetherDnsSet(netId, dnsServers);
                 mLog.log(String.format(