You don't need a dedicated Thunderbird Address Book. You also don't need to use Google Contacts. You can use whatever Contacts app you want (many of them are open source) syncing to any address book you want, including self-hosted address books using CardDav. Having a dedicated address book just for Thunderbird would not be a good idea, and it goes against Android design. If every app decided to have their own dedicated address book then you would have to manage one for Thunderbird, Telegram, Whatsapp, Line, and so on... I don't want to manage multiple address books, and Thunderbird should not be the first app to start this trend.