fix: prevent error when react extension was already created

[AlexanderEggers]

Summary:

Currently the react-native-gradle-plugin does not allow the "react" plugin extension to already exist when running its apply block. I had a use-case where I wanted to create a new gradle plugin which would take care of applying the react plugin including setting some of its options. Without the change in this PR, this would currently turn into a build failure.

Changelog:

[ANDROID] [FIXED] - prevent error when the "react" extension was already created by another gradle plugin

Test Plan:

[facebook-github-bot]

@zeyap has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	18,876,970	+4
android	hermes	armeabi-v7a	n/a	--
android	hermes	x86	n/a	--
android	hermes	x86_64	n/a	--
android	jsc	arm64-v8a	22,250,017	-3
android	jsc	armeabi-v7a	n/a	--
android	jsc	x86	n/a	--
android	jsc	x86_64	n/a	--

Base commit: d0cac87
Branch: main

[AlexanderEggers]

@zeyap I saw that you've imported this PR. I made some additional changes to get the CI green - just a few minor things I missed in my initial commit. Let me know if you notice anything.

[facebook-github-bot]

@zeyap merged this pull request in b9dad7f.

[NickGerleman]

This doesn't seem to compile.

FAILURE: Build failed with an exception.

* Where:
Build file '/root/react-native/packages/react-native/ReactAndroid/build.gradle.kts' line: 784

* What went wrong:
Script compilation errors:

  Line 784: react {
            ^ Unresolved reference: react

  Line 787:   libraryName = "rncore"
              ^ Unresolved reference: libraryName

  Line 788:   jsRootDir = file("../src")
              ^ Unresolved reference: jsRootDir

[AlexanderEggers]

@NickGerleman Were you testing with the latest commit in this PR? Can you show me how you tried to test that and I can give that a try as well

[AlexanderEggers]

All tests were also green, so I expected things to just work, no?

[NickGerleman]

This failed CircleCI tests that should run on PR.

[AlexanderEggers]

Oh okay.. well I will have another look at this (and will open a new PR soon) but I'm using this already as a patch and it is working fine for me locally.