I suppose it will be not possible to obtain tiles from a tile source with api key with the current version of osmdroid. The underlying library must be updated: see https://github.com/osmdroid/osmdroid/issues/573
Thanks but .....
I read that page but it refers to Thunderforest, not to CloudMade. I think they are different sites even if they actually offer the same maps.
I would, in one way or another, look at the maps WITHOUT the word "API Key Required"