Sync android extensions sample's build.xml with the one on the website.

The directory structure has been modified, the jar file used when
building an extension has been moved from "crosswalk-{version}/libs/" to
"crosswalk-{version}/template/libs/". To support legacy packages as
well, use in ant to find the xwalk_app_runtime_java.jar in crosswalk
package.

In addition to that, point to a more recent Crosswalk stable release (v8
instead of v6).

BUG=XWALK-2725
Merge pull request #20 from rakuco/update-extensions-example

Sync android extensions sample's build.xml with the one on the website.