Unify type and values of USE_FABRIC env variable with USE_CODEGEN

Summary:
This diff unifies the type and value of USE_FABRIC env variable  exposed in Gradle  with the USE_CODEGEN env variable

changelog: [internal] internal

Reviewed By: fkgozali

Differential Revision: D23145658

fbshipit-source-id: 9575f6b50c7a977254e364037d1417b3b1cdb607

RNTester/android/app/build.gradle

@@ -66,9 +66,6 @@ plugins {
  *   // Root dir for all JS files for the app. Defaults to `root` above.
  *   jsRootDir: "../..",
  *
- *   // Enable react-native-codegen during build time.
- *   enableCodegen: true,
- *
  *   // Enable Fabric at build time and runtime.
  *   enableFabric: true,
  *
@@ -88,8 +85,7 @@ project.ext.react = [
     hermesCommand: "../../../node_modules/hermes-engine/%OS-BIN%/hermesc",
     enableHermesForVariant: { def v -> v.name.contains("hermes") },
     jsRootDir: "$rootDir/RNTester",
-    enableCodegen: System.getenv('USE_CODEGEN'),
-    enableFabric: System.getenv("USE_FABRIC") ?: false,
+    enableFabric: (System.getenv('USE_FABRIC') ?: '0').toBoolean(),
 ]
 
 apply from: "../../../react.gradle"

ReactAndroid/build.gradle

@@ -34,9 +34,9 @@ def thirdPartyNdkDir = new File("$buildDir/third-party-ndk")
 def dependenciesPath = System.getenv("REACT_NATIVE_DEPENDENCIES")
 
 // The 'USE_FABRIC' environment variable will build Fabric C++ code into the bundle
-// USE_FABRIC=false will build RN excluding fabric
-// USE_FABRIC=true will build RN including fabric
-def enableFabric = System.getenv("USE_FABRIC") ?: "false"
+// USE_FABRIC=0 will build RN excluding fabric
+// USE_FABRIC=1 will build RN including fabric
+def enableFabric = (System.getenv('USE_FABRIC') ?: '0').toBoolean()
 
 // The Boost library is a very large download (>100MB).
 // If Boost is already present on your system, define the REACT_NATIVE_BOOST_PATH env variable