Cleanup imports and visibility in the `research/robust_aggregation` package.

docs/api_docs/python/_toc.yaml

docs/api_docs/python/index.md

docs/api_docs/python/tff/learning/framework.md

...hon/tff/learning/framework/build_stateless_broadcaster.md

docs/api_docs/python/tff/learning/framework/enhance.md

tensorflow_federated/python/learning/BUILD

tensorflow_federated/python/learning/framework/BUILD

tensorflow_federated/python/learning/framework/__init__.py

...orflow_federated/python/research/robust_aggregation/BUILD

...search/robust_aggregation/robust_federated_aggregation.py

...h/robust_aggregation/robust_federated_aggregation_test.py