So Amazon removed support in their Android SDK for MoPub, for unknown reasons. I can only assume they could not get it working.
I did get it working and for those in need, here is the link to my gist files:
https://gist.github.com/SRCApps/7fe00dc7ea178f8447910d4e18611c3f
Follow the comments in the files, or look into MoPub custom network. The files must be put in a folder called mopub in your application for it to work.
In MoPub you must use the correct location of the files as shown below: (I made a mistake, it should say AmazonInterstitial and not AmazonInt. AdRegistration is needed in all files to get it to work.
Tested and working for both banner and interstitial. Mopub dashboard will not report on the impression/request data though, however Amazon does.
If testing from the EU, it will not work as pointed out in other threads. Unless you use an Amazon approved CMP, you will only get impressions from non-eu countries and you can't even test ads without a VPN.
I hope eventually Amazon will allow us to use the MoPub GDPR data and allow us to forward it to the Amazon SDK (Please devs).
