[Android] Ensure onPageStarted won't called for iframe loading

Even for http/https iframe, onPageStarted would not be called. To hook
up iframe url loading, the embedder is able to override
shouldOverrideUrlLoading defined in XWalkClient.

TODO:
Consider to expose shouldOverrideUrlLoading as a public API?

BUG=https://crosswalk-project.org/jira/browse/XWALK-1780