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.
Have you ever herd of Aves? Kinda the opposite of "simple" in terms of feature set, but can be downloaded from F-Droid and is highly customizable. I really like it's tagging system/feature.