Use installed DBus libraries instead of generating them.

login_manager, power_manager, debugd and shill now expose a client
library with the generated dbus-proxies.h file and the system_api's
installed dbus-constants.h. This patch changes update_engine from
generating these DBus headers to use the installed client libraries.

The client libraries already include the service path string, so we
don't need to include system_api dbus-constants.h in most cases, unless
we actually use some parameter constants defined there.

BUG=b:23084776,b:23560718
TEST=./build_packages --board=link

Change-Id: Idb4501e784ebb5928c92902d114462be57d5826a
diff --git a/real_system_state.h b/real_system_state.h
index a51e6ef..31cf4da 100644
--- a/real_system_state.h
+++ b/real_system_state.h
@@ -21,12 +21,12 @@
 
 #include <memory>
 
+#include <debugd/dbus-proxies.h>
 #include <metrics/metrics_library.h>
 #include <policy/device_policy.h>
+#include <power_manager/dbus-proxies.h>
+#include <session_manager/dbus-proxies.h>
 
-#include "debugd/dbus-proxies.h"
-#include "login_manager/dbus-proxies.h"
-#include "power_manager/dbus-proxies.h"
 #include "update_engine/boot_control_interface.h"
 #include "update_engine/clock.h"
 #include "update_engine/connection_manager.h"