Add enable_nullify_catalyst_instance_on_destroy MC and gate setting mCatalystInstance to null in ReactContext

Summary:
Mostly for easing open-source migration and not making a backwards-incompatible change (yet), we'll set this to false by default. Every app can opt-in to this if wanted but it's not necessary. This change is part of experiments surrounding more-aggressive teardown for Fabric and Bridgeless mode.

Changelog: [Internal] - This has the effect of (by default) disabling the previous diff which caused ReactContext teardown to always set mCatalystInstance to null. Now that is opt-in behavior and off by default, so it's not longer a breaking change.

Reviewed By: mdvacca

Differential Revision: D18207302

fbshipit-source-id: 7acfc894415e966f652c7049849eef79c440a135

ReactAndroid/src/main/java/com/facebook/react/bridge/ReactContext.java

@@ -21,6 +21,7 @@ import com.facebook.infer.annotation.ThreadConfined;
 import com.facebook.react.bridge.queue.MessageQueueThread;
 import com.facebook.react.bridge.queue.ReactQueueConfiguration;
 import com.facebook.react.common.LifecycleState;
+import com.facebook.react.config.ReactFeatureFlags;
 import java.lang.ref.WeakReference;
 import java.util.concurrent.CopyOnWriteArraySet;
 
@@ -272,7 +273,9 @@ public class ReactContext extends ContextWrapper {
 
     if (mCatalystInstance != null) {
       mCatalystInstance.destroy();
-      mCatalystInstance = null;
+      if (ReactFeatureFlags.nullifyCatalystInstanceOnDestroy) {
+        mCatalystInstance = null;
+      }
     }
   }
 

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

@@ -61,4 +61,13 @@ public class ReactFeatureFlags {
    * CatalystInstanceImpl `destroy` method.
    */
   public static boolean useCatalystTeardownV2 = false;
+
+  /**
+   * When the ReactContext is destroyed, should the CatalystInstance immediately be nullified? This
+   * is the safest thing to do since the CatalystInstance shouldn't be used, and should be
+   * garbage-collected after it's destroyed, but this is a breaking change in that many native
+   * modules assume that a ReactContext will always have a CatalystInstance. This will be deleted
+   * and the CatalystInstance will always be destroyed in some future release.
+   */
+  public static boolean nullifyCatalystInstanceOnDestroy = false;
 }