[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
There is no other mime type contains both of them. And according to http://www.webstandards.org/learn/articles/askw3c/sep2003/, alternate mime type would be "application/xml".
But you are right, I use mimeType == "application/xhtml+xml" || mimeType == "application/xml" to make it more strict.