Allow configuration of 'allWarningsAsErrors' in gradle (#38930)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38930

As we migrate java to kotlin I noticed that we've introduced few warnings here and there.
I'm introducing a way to configure allWarningsAsErrors in gradle

changelog: [internal] internal

Reviewed By: NickGerleman

Differential Revision: D48238321

fbshipit-source-id: 745847bc6e6df94decc54afa8c9041552d91dfee

packages/react-native/ReactAndroid/build.gradle

@@ -441,6 +441,11 @@ def reactNativeArchitectures() {
     return value ? value.split(",") : ["armeabi-v7a", "x86", "x86_64", "arm64-v8a"]
 }
 
+def enableWarningsAsErrors() {
+    def value = project.getProperties().get("enableWarningsAsErrors")
+    return value != null ? value : false
+}
+
 task packageReactNdkLibsForBuck(type: Copy) {
     dependsOn("mergeDebugNativeLibs")
     // Shared libraries (.so) are copied from the merged_native_libs folder instead
@@ -484,6 +489,8 @@ android {
     kotlinOptions {
         // Using '-Xjvm-default=all' to generate default java methods for interfaces
         freeCompilerArgs = ['-Xjvm-default=all']
+        // Using -PenableWarningsAsErrors=true prop to enable allWarningsAsErrors
+        kotlinOptions.allWarningsAsErrors = enableWarningsAsErrors()
     }
 
     defaultConfig {