Fabric should be enabled or disabled app-wide without entrypoint-specific options

Summary:
Add new ReactFeatureFlag to enable or disable Fabric appwide.

Changelog: [Internal]

Reviewed By: fkgozali

Differential Revision: D30504714

fbshipit-source-id: f615d4c888a89f5401b0ff8f8305fdfbdf75fba3

ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java

@@ -25,6 +25,12 @@ public class ReactFeatureFlags {
    */
   public static volatile boolean useTurboModules = false;
 
+  /**
+   * Should this application use the new (Fabric) Renderer? If yes, all rendering in this app will
+   * use Fabric instead of the legacy renderer.
+   */
+  public static volatile boolean enableFabricRenderer = false;
+
   /**
    * After TurboModules and Fabric are enabled, we need to ensure that the legacy NativeModule isn't
    * isn't used. So, turn this flag on to trigger warnings whenever the legacy NativeModule system