[Runtime] Move application runtime& event extension under xwalk::application namespace.

Present ApplicationEventExtension & ApplicationRuntimeExtension are
located inside namespace "xwalk". Actually they are parts of
"application module". This PR moves the two inside namespace
"xwalk::application".
Merge pull request #1365 from shaochangbin/app-extension-namespace

[Runtime] Move application runtime& event extension under xwalk::application namespace.