Add a section for plugin extensions

The https://github.com/apache/cordova-lib/pull/119 add gradle reference from plugin to project. The cordova-lib will add the reference between the lines. It looks like:
// PLUGIN GRADLE EXTENSIONS START
apply from: 'libs/xwalk_core_library/xwalk.gradle'
// PLUGIN GRADLE EXTENSIONS END

The xwalk.gradle can set ext.multiarch=true to support multiple APKs by default, so add this section in here.