[application] Use Application for external URLs.

Use Application for external URLs. Rationals:
1) allows web pages to use some of XWalk APIs
2) makes in-browser testing simpler: no need in APIs like "Rungtime::SetGlobalObserverForTesting"
3) External URLs and applications share the same code path, which is better for both testing and maintainance

The  "app.launch.web_url" manifest key is used as application entry point for external URLs loading.

WIP: in-browser tests should be updated accordingly.