Un-bump AGP to 8.1.2 (#39958)

Summary:
Pull Request resolved: #39958

I'm reverting the change of AGP from 8.2 beta to 8.1 as we don't need 8.2
Using 8.2 beta forces us to use Android Studio beta, which is actually not necessary.

Changelog:
[Android] [Changed] - Bump AGP to 8.1.2

Reviewed By: yungsters

Differential Revision: D50016572

fbshipit-source-id: 6c36df0568a1f867dac3c335abbaabb990e55491

packages/react-native/gradle/libs.versions.toml

@@ -5,7 +5,7 @@ targetSdk = "34"
 compileSdk = "34"
 buildTools = "34.0.0"
 # Dependencies versions
-agp = "8.2.0-beta06"
+agp = "8.1.2"
 androidx-annotation = "1.6.0"
 androidx-appcompat = "1.6.1"
 androidx-autofill = "1.1.0"