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.
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'