Expose netId by adding getter API

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

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.
      *