Since the number of Android device types in the wild is literally in the thousands there’s no way to test on them all. I don’t think any developer, no matter how deep their pockets, could hope to test on more than a fraction of them. Given that it would be interesting to learn what other indie developers’ strategies are for testing on Android. My strategy is to test on 2-3 devices making sure that at least one of them is a low performance device with an older version of Android, and another is a more modern, mid- to hi- performance mainstream device with the most recent version of Android. If the app works as intended on both then I feel like I have a good chance it will work on most devices in between.
EDIT: To Naomi’s point as to if you should release a version that supports other android devices or just Kindle, my app was released for both and using analytics data I can see that Kindle to Non-Kindle sales is about 20:1. So maybe not critical at this point to support other Android devices if you don’t want to bother testing and just focus on the Kindle.