This feature was rolled out in July and Android app supports this feature as well. Instead of Bookmarks and adding articles to your reading list, now you can create a custom one. These lists can work as collections and you can add a description to them. Your lists can be also private and public so you can easily share them with others.
You can only add Medium posts to these lists so if you plan to use it as a creator, you need to export your editions to Medium itself first.