Summary:
Both changes are related to RocksJava:

1. Allow dependencies that are already present on the host system due to Maven to be reused in Docker builds.

2. Extend the `make clean-not-downloaded` target to RocksJava, so that libraries needed as dependencies for the test suite are not deleted and re-downloaded unnecessarily.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6318

Differential Revision: D19608742

Pulled By: pdillinger

fbshipit-source-id: 25e25649e3e3212b537ac4512b40e2e53dc02ae7