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
diff --git a/core/java/android/net/Network.java b/core/java/android/net/Network.java
index 3f56def..f12ba13 100644
--- a/core/java/android/net/Network.java
+++ b/core/java/android/net/Network.java
@@ -63,7 +63,7 @@
     /**
      * @hide
      */
-    @UnsupportedAppUsage
+    @SystemApi
     public final int netId;
 
     // Objects used to perform per-network operations such as getSocketFactory