OSM offline reverse geocoding for Android

I am looking for a way to get an address/es given location coordinates using offline maps.

As part of an academic project, I need reverse-geolocation functionalists for Android. It is essential that this is done offline (compared to Nominatim). More over I will use only sub-regions (France, England, etc.). I need that only for testing purposes and PoC(Proof of Concept) so I am looking for something easy to implement (currently I am using "GeocoderNominatim" from osmbonuspack (online service) so extending that for offline is preferable.

I am familiar with application such as OsmAnd (Open-source) which do that but are not structured as libraries. Implementing the aforementioned functionality following their design will be a huge overhead compared to the testing I want to do. However, this is still an option if there is no other way.

PS: Unfortunately, I was not able to find an answer to my problem in previous "Questions" on the "Help" page.

Thanks in advance.