Monerujo is open source and as reliable as it gets. It's just a wrapper for the regular Linux wallet anyway. The Monero subreddit recommends Monerujo and warns that Freewallet is a scam.

For long-term storage, you could create a written paper wallet using an offline address generator, so you don't need to connect to the blockchain at all. Of course you can't check your funds on a block explorer - that's a downside of Monero's anonymity in this case.