Update document to write an extension on Android

The directory structure has been modified, the jar
file will be used when building an extension has
been moved from "crosswalk-{version}/libs/" to
"crosswalk-{version}/template/libs/".
To support legacy packages as well, use <first>
in ant to find the xwalk_app_runtime_java.jar in
crosswalk package.

BUG=XWALK-2725
I tried this change with crosswalk-samples (`build.xml` should be roughly the same here and there) and it didn't work:

> /tmp/crosswalk-samples/extensions-android/xwalk-echo-extension-src/build.xml:65: /tmp/crosswalk-samples/extensions-android/xwalk-echo-extension-src/xwalk_app_template does not exist.

Are you sure you want "xwalk_app_template" here and not "${libs}/crosswalk-${crosswalk-version}"?