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