privetd: Implement privet/v3/commands/status

Privetd just forwards IDs to the buffet and buffet returns JSON
representation of the command.

BUG=brillo:430
TEST=unittest

Change-Id: I5698f9fc915ae32dc2b9be3cc35f8efe919ee4e3
Reviewed-on: https://chromium-review.googlesource.com/261641
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: 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 8473fe2..8b59b42 100644
--- a/buffet/dbus_bindings/org.chromium.Buffet.Manager.xml
+++ b/buffet/dbus_bindings/org.chromium.Buffet.Manager.xml
@@ -33,6 +33,11 @@
       <arg name="json_command" type="s" direction="in"/>
       <annotation name="org.chromium.DBus.Method.Kind" value="async"/>
     </method>
+    <method name="GetCommand">
+      <arg name="id" type="s" direction="in"/>
+      <arg name="json_command" type="s" direction="out"/>
+      <annotation name="org.chromium.DBus.Method.Kind" value="async"/>
+    </method>
     <method name="TestMethod">
       <arg name="message" type="s" direction="in"/>
       <arg name="echoed_message" type="s" direction="out"/>