Fix applyAppPlugin being accessed too early in the React App Gradle Plugin

[cortinico]

Summary:
While working on the NDK AGP Apis, I realized the the applyAppPlugin is accessed
too early inside the Gradle plugin. Specifically is accessed once the plugin is applied,
and the extension is not configured afterwards. This means that the extension is always set
the default values.

I'm fixing it moving it inside the project.afterEvaluate that was already need to access
the variant informations.

Changelog:
[Internal] [Changed] - Fix applyAppPlugin being accessed too early in the React App Gradle Plugin

Reviewed By: ShikaSD

Differential Revision: D31652984

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This pull request was exported from Phabricator. Differential Revision: D31652984

[pull-bot]

	Messages
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Generated by 🚫 dangerJS against 7a79062

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PR build artifact for 7a79062 is ready.
To use, download tarball from "Artifacts" tab in this CircleCI job then run yarn add <path to tarball> in your React Native project.

[analysis-bot]

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

Base commit: dfe42d6

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	8,981,012	+1,263,754
android	hermes	armeabi-v7a	8,507,581	+1,263,759
android	hermes	x86	9,399,869	+1,263,751
android	hermes	x86_64	9,365,534	+1,263,764
android	jsc	arm64-v8a	10,652,513	+1,016,275
android	jsc	armeabi-v7a	9,566,598	+1,016,277
android	jsc	x86	10,665,650	+1,016,279
android	jsc	x86_64	11,275,013	+1,016,299

Base commit: dfe42d6

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This pull request has been merged in 79e72e0.