docs: added Android P behavior change note to untagSocket reference

staged here:
https://android-dot-devsite.googleplex.com/reference/android/net/TrafficStats#untagSocket(java.net.Socket)

Test: make ds-docs

Exempt-From-Owner-Approval: Docs-only change

Bug: 110484513
Change-Id: I24575e6f4451a019b60c5bf60e17e14a928c6cc3

core/java/android/net/TrafficStats.java

@@ -329,6 +329,14 @@ public class TrafficStats {
 
     /**
      * Remove any statistics parameters from the given {@link Socket}.
+     * <p>
+     * In Android 8.1 (API level 27) and lower, a socket is automatically
+     * untagged when it's sent to another process using binder IPC with a
+     * {@code ParcelFileDescriptor} container. In Android 9.0 (API level 28)
+     * and higher, the socket tag is kept when the socket is sent to another
+     * process using binder IPC. You can mimic the previous behavior by
+     * calling {@code untagSocket()} before sending the socket to another
+     * process.
      */
     public static void untagSocket(Socket socket) throws SocketException {
         SocketTagger.get().untag(socket);