Summary:
In 2019-6-4 version of Docker Android image, we've added Android NDK r19c. So this PR will change CI to use NDK r19c.

I'll create a PR to website once this merged.

Note: I tried to build RN with NDK 19 while I was setting up my laptop after format, and it worked.

## Changelog

[Android] [Changed] - Bump Android NDK to r19c
Pull Request resolved: https://github.com/facebook/react-native/pull/25140

Differential Revision: D15631301

Pulled By: cpojer

fbshipit-source-id: b9a5600df1dadeba328932b694493960b242c2f7