WIP: Add an explicit NDK version to RNTester and ReactAndroid (#29987)

Summary:
When I try to run RNTester with Gradle the RNTester Required me to use **NDK 20.0.5594570**. I can't seem to find an explicit NDK version anywhere in ReactAndroid and RNTester. This PR Aims to add an explicit NDK version to RNTester and ReactAndroid.

![Screenshot from 2020-09-19 21-13-17](https://user-images.githubusercontent.com/8868908/93669563-444fcf00-fabf-11ea-8822-93264c5bb736.png)

## Changelog
[Android] [Added] - Add an explicit NDK version to RNTester and ReactAndroid.

Pull Request resolved: https://github.com/facebook/react-native/pull/29987

Test Plan: Build manually from RNTester

Reviewed By: fkgozali

Differential Revision: D23911371

Pulled By: ShikaSD

fbshipit-source-id: 2f297c73890c0eb0bfec0e2ba7ec5755b4d84243