Add public alternative for protectFromVpn

The method is called directly by VpnService#protect.

Bug: 171540887
Test: m
Change-Id: I7cbb1ef1301dcf8d27b2cc39c0431a0156fe3442
diff --git a/framework/src/android/net/NetworkUtils.java b/framework/src/android/net/NetworkUtils.java
index 9ccb04a..b5e8a61 100644
--- a/framework/src/android/net/NetworkUtils.java
+++ b/framework/src/android/net/NetworkUtils.java
@@ -91,7 +91,8 @@
      * this socket will go directly to the underlying network, so its traffic will not be
      * forwarded through the VPN.
      */
-    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
+    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553,
+            publicAlternatives = "Use {@link android.net.VpnService#protect} instead.")
     public static native boolean protectFromVpn(FileDescriptor fd);
 
     /**