Docker: Use the new community image, and update instructions. (#23723)

Summary:
The `Dockerfile.android` had fallen out of date. We now point to the new community image being used in the Circle CI `test_android` workflow. After spending an hour or two working with these Docker images, I saw some opportunities to clean up the docs as well.

[ANDROID] [Fixed] - Use new React Native Community Android base image for local Docker tests.
Pull Request resolved: https://github.com/facebook/react-native/pull/23723

Differential Revision: D14331737

Pulled By: hramos

fbshipit-source-id: cf42820021c997c3c06cc8b3671f67c9d2650ee5