First baby steps towards settings backup

This change adds a sketched outline of the backup system architecture, with
all of the major pieces represented other than client-side helpers for
specific types of data.  IBackupManager and BackupService are public so that
we can write test apps against SDK-domain symbols from the outset.

What code exists in this change hasn't been tested and may crash.  It's the
beginnings of the real implementation but of course is barely begun.

services/java/com/android/server/SystemServer.java

@@ -305,6 +305,13 @@ class ServerThread extends Thread {
                 Log.e(TAG, "Failure starting HeadsetObserver", e);
             }
 
+            try {
+                Log.i(TAG, "Starting Backup Service");
+                ServiceManager.addService(Context.BACKUP_SERVICE, new BackupManagerService(context));
+            } catch (Throwable e) {
+                Log.e(TAG, "Failure starting Backup Service", e);
+            }
+
             try {
                 Log.i(TAG, "Starting AppWidget Service");
                 appWidget = new AppWidgetService(context);