Making setFabricEnabled Flag optional for ReactFragments (#41211)

Summary:
Making setFabricEnabled Flag optional for ReactFragments

## Changelog:
[ANDROID][CHANGED] - Continuation of PR: 36263

Pull Request resolved: #41211

Test Plan:
Kotlin Code Snippet to test:
```
supportFragmentManager
  .beginTransaction()
  .add(android.R.id.content,
     ReactFragment.Builder()
       .setComponentName("componentName")
       .setFabricEnabled(true)
       .build())
  .commit()
```

Reviewed By: cipolleschi

Differential Revision: D50732345

Pulled By: cortinico

fbshipit-source-id: 812fe1abeb8c09334bd755ce3a29f55ce8b7ac7b

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/ReactFragment.java

@@ -181,7 +181,7 @@ public static class Builder {
     public Builder() {
       mComponentName = null;
       mLaunchOptions = null;
-      mFabricEnabled = null;
+      mFabricEnabled = false;
     }
 
     /**