Introduce socket stats tag for restore traffic

As well as system API for emplacing it.

Change-Id: If33fbd23552261efe3b00d75c22ff823564f43e6

core/java/android/net/TrafficStats.java

@@ -87,12 +87,21 @@ public class TrafficStats {
     public static final int TAG_SYSTEM_MEDIA = 0xFFFFFF02;
 
     /**
-     * Default tag value for {@link BackupManager} traffic.
+     * Default tag value for {@link BackupManager} backup traffic; that is,
+     * traffic from the device to the storage backend.
      *
      * @hide
      */
     public static final int TAG_SYSTEM_BACKUP = 0xFFFFFF03;
 
+    /**
+     * Default tag value for {@link BackupManager} restore traffic; that is,
+     * app data retrieved from the storage backend at install time.
+     *
+     * @hide
+     */
+    public static final int TAG_SYSTEM_RESTORE = 0xFFFFFF04;
+
     private static INetworkStatsService sStatsService;
 
     private synchronized static INetworkStatsService getStatsService() {
@@ -141,6 +150,16 @@ public class TrafficStats {
         setThreadStatsTag(TAG_SYSTEM_BACKUP);
     }
 
+    /**
+     * System API for restore-related support components to tag network traffic
+     * appropriately.
+     * @hide
+     */
+    @SystemApi
+    public static void setThreadStatsTagRestore() {
+        setThreadStatsTag(TAG_SYSTEM_RESTORE);
+    }
+
     /**
      * Get the active tag used when accounting {@link Socket} traffic originating
      * from the current thread. Only one active tag per thread is supported.