Make TrafficStats uid attribution @SystemApi

Anything that runs as a singleton may need to attribute traffic to
various client apps; in particular, backup transports need to do this.
Apropos of which, introduce a @SystemApi method specifically for that
purpose, setThreadStatsTagBackup().

Bug 16661321

Change-Id: Id5d22e28bdc68edb53f2a1fdba80b144fcbc61d2

core/java/android/net/TrafficStats.java

@@ -16,6 +16,7 @@
 
 package android.net;
 
+import android.annotation.SystemApi;
 import android.app.DownloadManager;
 import android.app.backup.BackupManager;
 import android.content.Context;
@@ -126,6 +127,16 @@ public class TrafficStats {
         NetworkManagementSocketTagger.setThreadSocketStatsTag(tag);
     }
 
+    /**
+     * System API for backup-related support components to tag network traffic
+     * appropriately.
+     * @hide
+     */
+    @SystemApi
+    public static void setThreadStatsTagBackup() {
+        setThreadStatsTag(TAG_SYSTEM_BACKUP);
+    }
+
     /**
      * Get the active tag used when accounting {@link Socket} traffic originating
      * from the current thread. Only one active tag per thread is supported.
@@ -160,11 +171,13 @@ public class TrafficStats {
      *
      * @hide
      */
+    @SystemApi
     public static void setThreadStatsUid(int uid) {
         NetworkManagementSocketTagger.setThreadSocketStatsUid(uid);
     }
 
     /** {@hide} */
+    @SystemApi
     public static void clearThreadStatsUid() {
         NetworkManagementSocketTagger.setThreadSocketStatsUid(-1);
     }