Start backing up wallpaper

This CL does the following:
   + adds an AbsoluteFileBackupHelper class for managing backup of files
     known by absolute path, not based off of the app's getFilesDir() root
   + bumps up the collection interval from its testing-only default of 1 second
     to 3 minutes
   + adds a SystemBackupAgent class to the main system package and names it as
     the android:backupAgent for the main OS package.  Right now this agent
     only backs up & restores the wallpaper file.
   + amend the Wallpaper Service to inform the Backup Manager when the wallpaper
     changes.

On the subject of the 3-minute collection interval before the backup actually
occurs:  this can be short-circuited from an adb shell.  Running the command
'bmgr run' will cause the Backup Manager to kick off any pending backup
operations immediately.
diff --git a/services/java/com/android/server/BackupManagerService.java b/services/java/com/android/server/BackupManagerService.java
index fdd4617b..efa6179 100644
--- a/services/java/com/android/server/BackupManagerService.java
+++ b/services/java/com/android/server/BackupManagerService.java
@@ -76,8 +76,7 @@
     private static final boolean DEBUG = true;
 
     // Default time to wait after data changes before we back up the data
-    private static final long COLLECTION_INTERVAL = 1000;
-    //private static final long COLLECTION_INTERVAL = 3 * 60 * 1000;
+    private static final long COLLECTION_INTERVAL = 3 * 60 * 1000;
 
     private static final int MSG_RUN_BACKUP = 1;
     private static final int MSG_RUN_FULL_BACKUP = 2;
@@ -360,7 +359,8 @@
         if (DEBUG) {
             Log.v(TAG, "Adding " + targetPkgs.size() + " backup participants:");
             for (PackageInfo p : targetPkgs) {
-                Log.v(TAG, "    " + p + " agent=" + p.applicationInfo.backupAgentName);
+                Log.v(TAG, "    " + p + " agent=" + p.applicationInfo.backupAgentName
+                        + " uid=" + p.applicationInfo.uid);
             }
         }