More generous access to DestructorThread.Destructor

Summary:
In order to extend / use  `DestructorThread.Destructor` outside of `com.facebook.jni`, we need access modifiers to be less strict:

- `Destructor#Destructor()`: package protected -> public
- `Destructor#destruct()`: package protected -> protected

This will enable Yoga to move from finalizers to `DestructorThread.Destructor` without having to buy into `HybridData` completely.

Reviewed By: cjhopman

Differential Revision: D16182362

fbshipit-source-id: ad616c403df8e7c1e3d751131cfb7a9cfe62cf24

ReactAndroid/src/main/java/com/facebook/jni/DestructorThread.java

@@ -31,7 +31,7 @@ public abstract static class Destructor extends PhantomReference<Object> {
     private Destructor next;
     private Destructor previous;
 
-    Destructor(Object referent) {
+    public Destructor(Object referent) {
       super(referent, sReferenceQueue);
       sDestructorStack.push(this);
     }
@@ -41,7 +41,7 @@ private Destructor() {
     }
 
     /** Callback which is invoked when the original object has been garbage collected. */
-    abstract void destruct();
+    protected abstract void destruct();
   }
 
   /** A list to keep all active Destructors in memory confined to the Destructor thread. */
@@ -84,7 +84,7 @@ public void run() {
 
   private static class Terminus extends Destructor {
     @Override
-    void destruct() {
+    protected void destruct() {
       throw new IllegalStateException("Cannot destroy Terminus Destructor.");
     }
   }

ReactAndroid/src/main/java/com/facebook/jni/HybridData.java

@@ -63,7 +63,7 @@ public static class Destructor extends DestructorThread.Destructor {
     }
 
     @Override
-    void destruct() {
+    protected final void destruct() {
       // When invoked from the DestructorThread instead of resetNative,
       // the DestructorThread has exclusive ownership of the HybridData
       // so synchronization is not necessary.