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/dbus_constants.h b/dbus_constants.h
index e048b8b..e9b66ab 100644
--- a/dbus_constants.h
+++ b/dbus_constants.h
@@ -17,6 +17,9 @@
 #ifndef UPDATE_ENGINE_DBUS_CONSTANTS_H_
 #define UPDATE_ENGINE_DBUS_CONSTANTS_H_
 
+// TODO(deymo): Remove this file once all clients use "dbus-constants.h"
+// instead. We need to remove it from the ebuild as well.
+
 namespace chromeos_update_engine {
 
 static const char* const kUpdateEngineServiceName = "org.chromium.UpdateEngine";