Update to the above:
Now I'm more convinced (about 50% sure) that it's an ESRI problem: I can see the .mmpk file in the Assets folder on the .apk file, i.e., it is being copied (right next to Assets.txt, I made that Always Copy just to be able to check). Now Assets.txt has a path of /Assets.txt, so it follows that MahouRivieraTrails.mmpk ought to have a path of /MahouRivieraTrails.mmpk, and it does. But when I run this code:
// Define the path to the mobile map package.
string pathToMobileMapPackage = Path.Combine(Environment.CurrentDirectory, "MahouRivieraTrails.mmpk");
MobileMapPackage mahouRivieraTrails_MobileMapPackage = new MobileMapPackage(pathToMobileMapPackage);
// Load the mobile map package.
await mahouRivieraTrails_MobileMapPackage.LoadAsync();
// Show the first map in the mobile map package.
this.Map = mahouRivieraTrails_MobileMapPackage.Maps.FirstOrDefault();
it throws an exception on the LoadAsync() line, the error being:
System.IO.FileNotFoundException: File not found: /MahouRivieraTrails.mmpk
I'm desperate, and going to file a bug report, but if anybody has any suggestions (apart from give up and go back to Swift), I'd really, really like to hear from you!