Merge the two Gradle Plugins (#32177)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/32177

This diff merges the `react-native-gradle-plugin` and the `react-codegen/android` into a single plugin.
This will allow us to iterate faster on a single plugin, will create less confusion for our users (`react` vs `reactApp`)
and will help us avoid race conditions when the two plugins are applied together (as we will control the whole lifecycle of it).

Changelog:
[Internal] [Changed] - Merged the two Gradle Plugins

allow-large-files

Reviewed By: ShikaSD

Differential Revision: D30765147

fbshipit-source-id: fcb02a181c7d900daa514107c637d0ee0225976c