Expose UdpEncapsulationSocket.getResourceId()

ConnectivityManager.createSocketKeepalive() will need to get
the socket resource ID as a parameter for creating a new
NattSocketKeepalive. ConnectivityManager is a part of incoming
ConnectivityService mainline, so expose getResourceId() as an
API since the hidden APIs are not accessible for a mainline
module. This API should not be exposed to apps, so make it a
MODULE_LIBRARIES system API.

Bug: 172183305
Test: make update-api
Change-Id: Ic0722352ea186fcb18a2d91cc3969f771fde9e86

core/java/android/net/IpSecManager.java

@@ -15,6 +15,8 @@
  */
 package android.net;
 
+import static android.annotation.SystemApi.Client.MODULE_LIBRARIES;
+
 import static com.android.internal.util.Preconditions.checkNotNull;
 
 import android.annotation.NonNull;
@@ -628,7 +630,7 @@ public final class IpSecManager {
         }
 
         /** @hide */
-        @VisibleForTesting
+        @SystemApi(client = MODULE_LIBRARIES)
         public int getResourceId() {
             return mResourceId;
         }