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_bindings/org.chromium.UpdateEngineInterface.dbus-xml b/dbus_bindings/org.chromium.UpdateEngineInterface.dbus-xml
index a632608..deacabd 100644
--- a/dbus_bindings/org.chromium.UpdateEngineInterface.dbus-xml
+++ b/dbus_bindings/org.chromium.UpdateEngineInterface.dbus-xml
@@ -16,7 +16,7 @@
     <method name="AttemptUpdateWithFlags">
       <arg type="s" name="app_version" direction="in" />
       <arg type="s" name="omaha_url" direction="in" />
-      <!-- See AttemptUpdateFlags enum in dbus_constants.h. -->
+      <!-- See AttemptUpdateFlags enum in update_engine/dbus-constants.h. -->
       <arg type="i" name="flags" direction="in" />
     </method>
     <method name="AttemptRollback">