Split DBus client proxies into separated headers.

Each daemon we talk to should expose its own DBus library. This patch
splits the DBus proxies we generate from other daemon's .xml files into
separated header files so we can then replace them with libraries.

BUG=b:23084776
TEST=FEATURES=test emerge-link update_engine

Change-Id: Idbf1671eb7fc4259d493dbe2c8bcc8dad5b8784f
diff --git a/system_state.h b/system_state.h
index 8c26c51..4d4c74a 100644
--- a/system_state.h
+++ b/system_state.h
@@ -17,7 +17,7 @@
 #ifndef UPDATE_ENGINE_SYSTEM_STATE_H_
 #define UPDATE_ENGINE_SYSTEM_STATE_H_
 
-#include "update_engine/dbus_proxies.h"
+#include "power_manager/dbus-proxies.h"
 
 class MetricsLibraryInterface;