Update .gitignore to ignore ReactAndroid generated Gradle files (#28014)

Summary:
When building RNAndroid, Gradle files are generated. Added them to `.gitignore`. If it is ideal to have them in the repo I can commit them.

## Changelog

[Internal] [Fixed] - Added ReactAndroid generated Gradle files to .gitignore
Pull Request resolved: #28014

Test Plan: Build ReactAndroid, generated Gradle files no longer show up in git changes.

Differential Revision: D19873998

Pulled By: mdvacca

fbshipit-source-id: 29af7612df611bba1e83f4afac8692e1aa6df515

.gitignore

@@ -28,6 +28,9 @@ project.xcworkspace
 /RNTester/android/app/gradlew
 /RNTester/android/app/gradlew.bat
 /ReactAndroid/build/
+/ReactAndroid/gradle/
+/ReactAndroid/gradlew
+/ReactAndroid/gradlew.bat
 
 # Buck
 .buckd