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.cc b/real_system_state.cc
index 5477dd6..c89f297 100644
--- a/real_system_state.cc
+++ b/real_system_state.cc
@@ -18,7 +18,6 @@
 
 #include <base/files/file_util.h>
 #include <base/time/time.h>
-#include <chromeos/dbus/service_constants.h>
 
 #include "update_engine/boot_control_chromeos.h"
 #include "update_engine/constants.h"
@@ -28,9 +27,9 @@
 namespace chromeos_update_engine {
 
 RealSystemState::RealSystemState(const scoped_refptr<dbus::Bus>& bus)
-    : debugd_proxy_(bus, debugd::kDebugdServiceName),
-      power_manager_proxy_(bus, power_manager::kPowerManagerServiceName),
-      session_manager_proxy_(bus, login_manager::kSessionManagerServiceName),
+    : debugd_proxy_(bus),
+      power_manager_proxy_(bus),
+      session_manager_proxy_(bus),
       shill_proxy_(bus),
       libcros_proxy_(bus) {
 }