- update CLI to alpha.20 to fix Android tests (#24869)

Summary:
PR https://github.com/facebook/react-native/pull/24843 broke Android tests because of a regression we introduced in terms of passing `reporter` argument to Metro config. This was fixed in latest `alpha.20` release of CLI

## Changelog

[General] [Fix] - update CLI to alpha.20 to fix Android tests
Pull Request resolved: https://github.com/facebook/react-native/pull/24869

Reviewed By: rickhanlonii

Differential Revision: D15355144

Pulled By: cpojer

fbshipit-source-id: faafd8098c708845264b7164557076bce45ea332