shill: tighten up constness of PropertyStore

BUG=chromium-os:20543
TEST=unittest

Change-Id: Ifa86f82a883b3c3ec61ce10f8d82564b9a036b04
Reviewed-on: http://gerrit.chromium.org/gerrit/7901
Tested-by: mukesh agrawal <quiche@chromium.org>
Reviewed-by: Chris Masone <cmasone@chromium.org>
diff --git a/device.h b/device.h
index 995517d..5e9e068 100644
--- a/device.h
+++ b/device.h
@@ -87,7 +87,8 @@
   // instance.
   const std::string &UniqueName() const;
 
-  PropertyStore *store() { return &store_; }
+  PropertyStore *mutable_store() { return &store_; }
+  const PropertyStore &store() const { return store_; }
 
   bool Load(StoreInterface *storage);
   bool Save(StoreInterface *storage);