[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
I did exactly the same since the kTizenNamespacePrefix key was outside both key namespaces. It would be good to move these inside. However I don't think it is good idea to complicate this patch. The kTizenNamespacePrefix key is used in few other places not modified by this patch yet. What do you think @halton?