There is, but it's Java - they would have to write JNI. I don't think there's anything wrong with using CURL. Should probably be a bit careful about calling it "native" though.
I just don't really see the point to how they've gone about this. Cpprestsdk is already structured in a way where writing different native client impls for each platform is part of the design. So, rather than adding more native implementations there, they've put another layer on top of cpprestsdk, and added some new client implementations in this new library, and for others it calls down and uses cpprestsdk's existing implementations.
This seems to fragment things more than if they had simply contributed to (or even forked) Microsoft's project.
I am surprised about Android though. Is there really no built-in HTTP client in Android that they could use?