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/fake_shill_proxy.h b/fake_shill_proxy.h
index 4feb2b8..8daf7c2 100644
--- a/fake_shill_proxy.h
+++ b/fake_shill_proxy.h
@@ -22,9 +22,9 @@
 #include <string>
 
 #include <base/macros.h>
+#include <shill/dbus-proxies.h>
+#include <shill/dbus-proxy-mocks.h>
 
-#include "shill/dbus-proxies.h"
-#include "shill/dbus-proxy-mocks.h"
 #include "update_engine/shill_proxy_interface.h"
 
 namespace chromeos_update_engine {