Summary:
Avoids re-downloading the .tar.gz files for the static build of RocksJava if they are already present.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4769

Differential Revision: D13491919

Pulled By: ajkr

fbshipit-source-id: 9265f577e049838dc40335d54f1ff2b4f972c38c