Move index.xml to index.html and move up two directories

My first thought was that there was some issue with having
assets/ in the path to the HTML file, which is why it wasn't loading
correctly.

Then I noticed that it was an XML file, not an HTML file. I don't
know whether this might cause some issues, but I'd use a .html suffix
anyway.

I did try it again in assets/www/, but if you want to use that
approach, I would suggest moving other web assets (the js/ directory for example)
down into the assets/www/ directory as well. I'm not sure whether the
path resolution will work correctly if you're trying to do "/js/" from a file
in assets/www/ for example. Safer to just use relative paths.
Refer to the correct Java class

The Java class referred to was org.crosswalkproject.sample.Echo,
but it's actually org.crosswalkproject.sample.echo.Echo.
Alert using the echo extension
The correct variable is 'extension' not 'extensions'