AU: Implement getting of tracks through GetTrack.

Add SetTrack and GetTrack to UpdateEngine.xml as well.

BUG=chromium-os:8104
TEST=tested on device with update_engine_client

Change-Id: I10ef2552e9280524b3a8d5f232d104a81a114e06

Review URL: http://codereview.chromium.org/4181001
diff --git a/dbus_service.h b/dbus_service.h
index b802fae..4fe17e9 100644
--- a/dbus_service.h
+++ b/dbus_service.h
@@ -59,6 +59,10 @@
                                           int64_t* new_size,
                                           GError **error);
 
+gboolean update_engine_service_get_track(UpdateEngineService* self,
+                                         gchar** track,
+                                         GError **error);
+
 gboolean update_engine_service_reboot_if_needed(UpdateEngineService* self,
                                                 GError **error);