[Tizen] Fix widget namespace value validation.

According to widget W3C spec (http://www.w3.org/TR/widgets/#namespace), the namespace cannot be absent or invalid.
If namespace is absent or wrong, the widget should be treated as invalid package and installation should be failed.

BUG=XWALK-2978
Any reason not put `kW3CNamespaceKey` and `kW3CNamespacePrefix` under namespace `application_widget_keys`?