Don't depend on echo/curl/sha256sum for the builder

[karalabe]

To build the Android package (yarn run android), we pull in a remote .aar binary as a first build step (you can manually do it with yarn run android-archive). The code doing this is at https://github.com/coronanet/rn-coronanet/blob/master/package.json#L10 .

This is a bit icky as it relies on the availability of echo, curl, and sha256sum, which may not be installed; will definitely not work on Windows; and not sure if the pipes and logical groupings work in macos and/or non-bash.

Would be nice to replace this with a .js code (?) that does the equivalent, without requiring any external dependencies.