Flatten the react-native-codegen included build.

[cortinico]

Summary:
This Diff simplifies the codegen Gradle build.
Previously the build used to have a 2-level nesting of included build.

Turns out that the react-native-codegen/android/build.gradle build
is just providing a task and including an inner build that contains the codegen
Gradle plugin. I've moved such plugin to the outer build.

This will also make sure that the Gradle plugin files are properly index by the
IDE when opening the build (as nested included build are not yet supported).

Changelog:
Internal - Flatten the react-native-codegen Gradle included build

Reviewed By: fkgozali

Differential Revision: D30227784

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D30227784

[pull-bot]

	Messages
📖	📋 Missing Changelog - Can you add a Changelog? To do so, add a "## Changelog" section to your PR description. A changelog entry has the following format: [CATEGORY] [TYPE] - Message. CATEGORY may be: General iOS Android JavaScript Internal (for changes that do not need to be called out in the release notes) TYPE may be: Added, for new features. Changed, for changes in existing functionality. Deprecated, for soon-to-be removed features. Removed, for now removed features. Fixed, for any bug fixes. Security, in case of vulnerabilities. MESSAGE may answer "what and why" on a feature level. Use this to briefly tell React Native users about notable changes.
📖	📋 Missing Test Plan - Can you add a Test Plan? To do so, add a "## Test Plan" section to your PR description. A Test Plan lets us know how these changes were tested.
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Generated by 🚫 dangerJS against e2b391b

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: ae494e7

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	9,241,377	+0
android	hermes	armeabi-v7a	8,753,209	+0
android	hermes	x86	9,684,376	+0
android	hermes	x86_64	9,652,583	+0
android	jsc	arm64-v8a	10,877,528	+0
android	jsc	armeabi-v7a	9,780,611	+0
android	jsc	x86	10,915,928	+0
android	jsc	x86_64	11,525,640	+0

Base commit: ae494e7

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D30227784

[facebook-github-bot]

This pull request has been merged in 1f8c560.