Fix regression of improper assets copy (revert #24518 #24778) (#25363)

Summary:
Pull requests https://github.com/facebook/react-native/issues/24518 #24778 make Gradle copy all **generated** assets and resources into `android/app/src/res`, which is a bad behavior, because `src/res` goes into version control and should hold only those **original** resource files.

These changes in https://github.com/facebook/react-native/issues/24518 #24778 were merged into 0.60.0-rc release and cause regression.

This pull request will:

- Revert pull requests https://github.com/facebook/react-native/issues/24518 #24778
- Close https://github.com/facebook/react-native/issues/25325

## Changelog

[Android] [Fixed] - Fix regression of improper assets copy (revert https://github.com/facebook/react-native/issues/24518 #24778)
Pull Request resolved: https://github.com/facebook/react-native/pull/25363

Test Plan: It is a revert pull request and the reverted script should work the same as it has in 0.59.x.

Differential Revision: D15963329

Pulled By: cpojer

fbshipit-source-id: 5619a318dbdb40e816e37b6e37d4fe32caa46e9e