I added a comment with my recommended solution to this problem. However, here is the answer to your question:
Of course, you can unzip the APK, swap assets, and then create a new APK. However, you will need to sign the APK again with the correct key.
If you create applications yourself, you can simply create several APKs, each of which contains a different asset file. However, you will have to distribute these APKs manually (i.e., they cannot be downloaded to Google Play) because each of them has the same package name, and you cannot have multiple applications in the application store with the same package name.
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