Fix some @hide usage in Tethering

Using alternative way to replace some @hide API.

Bug: 144814072
Test: build, flash, boot
      atest TetheringTests

Change-Id: I1e12d69db1ad91dff553e142e17c6a70808e1639

Tethering/src/android/net/util/TetheringUtils.java

@@ -39,4 +39,11 @@ public class TetheringUtils {
      */
     public static native void setupRaSocket(FileDescriptor fd, int ifIndex)
             throws SocketException;
+
+    /**
+     * Read s as an unsigned 16-bit integer.
+     */
+    public static int uint16(short s) {
+        return s & 0xffff;
+    }
 }