buffet: Add DeviceId DBus property

Privetd needs to be notified when GCD ID is changed.

CQ-DEPEND=CL:260681
BUG=brillo:572
TEST=manual, register device using privetd, edit id in buffet.state, restart
             buffet, check if privetd has new id

Change-Id: I590ee22f134261711f78da75a4766043e0867bf5
Reviewed-on: https://chromium-review.googlesource.com/260303
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
diff --git a/buffet/dbus_bindings/org.chromium.Buffet.Manager.xml b/buffet/dbus_bindings/org.chromium.Buffet.Manager.xml
index 2e23802..7ec49a7 100644
--- a/buffet/dbus_bindings/org.chromium.Buffet.Manager.xml
+++ b/buffet/dbus_bindings/org.chromium.Buffet.Manager.xml
@@ -56,5 +56,10 @@
                                  valid.
       </tp:docstring>
     </property>
+    <property name="DeviceId" type="s" access="read">
+      <tp:docstring>
+        GCD ID if the device is registered or empty otherwise.
+      </tp:docstring>
+    </property>
   </interface>
 </node>