More details on what exactly does your project would be welcome. My understanding is that you want to sell a library (a set of components/tools developers can integrate in their app) without disclosing the source.
As you stated, you can't include assets/resources in Jar files.
An Android Library Project would force you to let the source code be "visible". Proper licensing terms could help you put legal fences around source code usage but... well, you know what people do with licenses... Maybe some code obfuscation (not on the public visible methods) could help.
I'm not sure about how "shared libraries" included with <uses-library> work, but the examples have seen are system dependent (maps API available only on "official" Google accepted devices, Sprint Evo front facing camera...) and might require to be built within the system. Maybe some root privilege could help adding one, but this would have to be done on EACH device where applications using your library would want to run...
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