RNTester Android: always compile Fabric files

Summary:
This commit makes both `:ReactAndroid` and `:rn-tester:android:app` always compile in Fabric codegen outputs. However, one may still enable/disable Fabric at runtime by setting `USE_FABRIC` env var (set to 1 or 0, default is 0).

Note that we can't register custom components specific to the app, yet, so only the components in react-native github repo is covered by this commit.

RNTester doesn't enable Fabric by default yet due to known UI bugs that haven't been addressed yet.

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D25674311

fbshipit-source-id: 8db660c959319250ebc683c84076677cf6489e94

scripts/generate-native-modules-specs-cli.js

@@ -26,9 +26,7 @@ const path = require('path');
 const USE_FABRIC = process.env.USE_FABRIC != null && !!process.env.USE_FABRIC;
 
 const GENERATORS = {
-  android: USE_FABRIC
-    ? ['componentsAndroid', 'modulesAndroid']
-    : ['modulesAndroid'],
+  android: ['componentsAndroid', 'modulesAndroid'],
   ios: ['modulesIOS'],
 };