My colleagues built Nostrino, a Kotlin SDK for Nostr. Though it hasn’t had activity for 6 months and I don’t know what state it’s in. nostr:npub1sdnq9yr3kwzaauhylwty6ttnum6zgcf34s0lwcythjcew8zss8fqlm45zq nostr:npub1nj0v6ly23sc5ftjghap9kevjerjncwzlmqehd48lklm2cxshh74sge88gs 
https://github.com/cashapp/nostrino

There’s also the Rust SDK, which has bindings to Kotlin. Unsure about its state either. nostr:npub1drvpzev3syqt0kjrls50050uzf25gehpz9vgdw08hvex7e0vgfeq0eseet 
https://github.com/rust-nostr/nostr

Lastly, nostr:npub1gcxzte5zlkncx26j68ez60fzkvtkm9e0vrwdcvsjakxf9mu9qewqlfnj5z might be able to offer some insight on if and how to base your work off Amethyst.