Equirectangular projection is the right call for a mobile panorama viewer since it's the standard format most cameras and drones output directly. The Wikimedia Commons reference is actually useful given how easy it is to end up with a phone full of stitched panoramas you never look at again. Does the app handle local file imports or is it gallery-only?
F-Droid
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
IzzyOnDroid is an F-Droid style repository for Android apps, provided by IzzyOnDroid. Applications in this repository are official binaries built by the original application developers, taken from their resp. repositories (mostly Github).
Yes it handles opening local files, or you can share an online image to Panoramicon.
I downloaded each of the Wikimedia pictures and then open them from within a local directory, by clicking on 'OPEN IMAGE'.

Now if they could just add taking photospheres, I would be on board. I wish Google kept supporting and improving on themin their Camera apps :(