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/shill_proxy.h b/shill_proxy.h
index 3e62793..1fbcd2b 100644
--- a/shill_proxy.h
+++ b/shill_proxy.h
@@ -22,8 +22,8 @@
 
 #include <base/macros.h>
 #include <dbus/bus.h>
+#include <shill/dbus-proxies.h>
 
-#include "shill/dbus-proxies.h"
 #include "update_engine/shill_proxy_interface.h"
 
 namespace chromeos_update_engine {