Add workaround for Android Gradle Plugin 3.2 change to asset dir (#21409)

Summary:
Android Gradle Plugin 3.2 uses a new intermediates/merged_assets directory instead of intermediates/assets. This workaround copies the javascript bundle to both directories for compatibility purposes.

Fixes #21132
Fixes #18357
Pull Request resolved: https://github.com/facebook/react-native/pull/21409

Differential Revision: D10141860

Pulled By: hramos

fbshipit-source-id: 0fb20fcec67ec2bfd7a8d9052599bbc70464b466