Add support to update_engine_client for -app_version and -omaha_url.

These options prevent auto-detection of these parameters.
Note that this CL makes the check_for_update DBus method obsolete from
the client's point of view.

BUG=4593
TEST=unit tests, gmerged on device and tried the client with different options.

Review URL: http://codereview.chromium.org/3048008
diff --git a/dbus_service.h b/dbus_service.h
index 40cb987..4b760ce 100644
--- a/dbus_service.h
+++ b/dbus_service.h
@@ -57,6 +57,11 @@
 gboolean update_engine_service_check_for_update(UpdateEngineService* self,
                                                 GError **error);
 
+gboolean update_engine_service_attempt_update(UpdateEngineService* self,
+                                              gchar* app_version,
+                                              gchar* omaha_url,
+                                              GError **error);
+
 gboolean update_engine_service_emit_status_update(
     UpdateEngineService* self,
     gint64 last_checked_time,