Reduce visibility of ShadowNodeRegistry

Summary:
In an attempt to reduce footprint of React Native Android public APIs we are reducing visibility of classes and interfaces that are not meant to be used publicly OR are public but have no usages.
As part of our analysis, which involved looking for usages inside the Meta codebase and code search in OSS, we've detected that this class/interface is public but it's not used from other packages.

If you are using this class or interface please comment in this PR and we will restate the public access.

bypass-github-export-checks

changelog: [Android][Changed] Reducing visibility of ShadowNodeRegistry

Reviewed By: arushikesarwani94

Differential Revision: D49803275

fbshipit-source-id: a2796a1e125f0399418aa56cce6eafe7c0509eeb

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

@@ -16,7 +16,7 @@
  * Simple container class to keep track of {@link ReactShadowNode}s associated with a particular
  * UIManagerModule instance.
  */
-public class ShadowNodeRegistry {
+class ShadowNodeRegistry {
 
   private final SparseArray<ReactShadowNode> mTagsToCSSNodes;
   private final SparseBooleanArray mRootTags;