[Android] Fix file://*/*.xhtml will be opened in default browser.

Because the minitype of xhtml file is "application/xhtml+xml".
It's intercepted by XWalkNavigationHandler to not handle in Crosswalk.
The default behavior for xhtml should be the same as html file, that
is loaded in the Crosswalk no matter with file schema or http schema.

BUG=XWALK-2912
Is it safe to use `contains()` here (ie. do you really want to match either of those, or even those strings as part of another mimetype instead of only "application/xhtml+xml")?