Summary:
Instead of sourcing-in a .gradle file to setup codegen tasks in Gradle, let's define a proper `com.facebook.react.codegen` Gradle plugin, so that any Gradle project (lib/app) can include it via:
```
plugins {
id 'com.facebook.react.codegen'
}
```
The idea (not yet implemented in this commit) is to then allow those projects to add this section in the projects:
```
codegen {
enableCodegen = ...
jsRootDir = ...
}
```
This is more scalable and less hacky.
Important notes:
* The Gradle plugin should be prepared during the build, we're not going to publish it to Maven or other repo at this point.
* This setup is inspired by composite build setup explained here: https://ncorti.com/blog/gradle-plugins-and-composite-builds
* All android specific setup is added under `packages/react-native-codegen/android/` dir, but long term, we may want to move it up to `packages/react-native-codegen/` along side setup for other platforms.
* As part of this setup, the plugin will have an option (to be validated) to produce Java specs using https://github.com/square/javapoet
* This is the same library already used for React Native Android annotation processors
* This generator will not deal with parsing Flow types into schema, it will just takes in the schema and produce Java code
* We're evaluating whether JavaPoet is a better choice for Java code generation long term, vs building it in JS via string concatenation: https://github.com/facebook/react-native/blob/master/packages/react-native-codegen/src/generators/modules/GenerateModuleJavaSpec.js
* This commit produces a sample Java code, not the actual codegen output
Changelog: [Internal]
To try this out, run this Gradle task:
```
USE_CODEGEN=1 ./gradlew :ReactAndroid:generateJava
```
Reviewed By: JoshuaGross, mdvacca
Differential Revision: D22917315
fbshipit-source-id: 0b79dba939b73ff1305b4b4fd86ab897c7a48d53
83edd0c5fe