Simple Mobile apps have been very popular among FOSS enthusiasts. I've personally been using the Gallery, Contacts and the Phone app since a few years now. It's a shame that it has come to this, will be on the lookout for their forks.
Because freeloaders who call themselves "open source enthusiasts" aren't really good at supporting people who create them. It's often quite the opposite. Rude, entitled, and ungrateful bunch of whiners that want everything free and some more. Then act surprised after their favorite opensource app is shutdown, sold or archived.
This guy even had paid apps on the playstore so whoever says "but opensouce doesn't mean you can't sell em" can fuck right off.
I've been maintaining a free app for over 6 years now. It's a lot of work. It's not like you have crunch time, but it's like you can go from just doing light maintenance to suddenly needing to drop 20 hours on it within a week on a dimes notice if there are major changes to certain dependencies. On top of that, the consistency is difficult. I will get bored and not work on it for months but I still need to keep up with the latest changes or else compatibility might break.
Having monetary incentive really helps with the boring work. The work that you need to do to keep the app going. And not necessarily the feature work which is usually a lot more fun.
That one doesnt have the security vulnerability and I think it is also better. But they are pretty much equal.
Simple Gallery - Aves
I prefer Simple Gallery though, it is pretty great
Simple dialer - Welefon
Very basic, I personally use ACR phone for call recording, with disabled internet permissions. The Dev asked on Reddit about opensourcing the App and got negative responses, so maybe unlikely.
Simple Keyboard - Florisboard
Currently unmaintained but awesome. I hope it gets revived.