shill: SIM property is an object path, not a string

Add DBusProperties::GetObjectPath() helper to accomodate this.

BUG=None
TEST=Run on a system with modemmanager-next and a GSM modem; see if it crashes.

Change-Id: I55df239b5b69e260fb1ad1d7983154ee717727b2
Reviewed-on: https://gerrit.chromium.org/gerrit/20516
Reviewed-by: Jason Glasgow <jglasgow@chromium.org>
Commit-Ready: Nathan J. Williams <njw@chromium.org>
Tested-by: Nathan J. Williams <njw@chromium.org>
diff --git a/dbus_properties.h b/dbus_properties.h
index 0a290ef..a88ea76 100644
--- a/dbus_properties.h
+++ b/dbus_properties.h
@@ -21,6 +21,10 @@
                         const std::string &key,
                         std::string *value);
 
+  static bool GetObjectPath(const DBusPropertiesMap &properties,
+                            const std::string &key,
+                            std::string *value);
+
   static bool GetUint32(const DBusPropertiesMap &properties,
                         const std::string &key,
                         uint32 *value);