Make writable arrays and maps only shallowly writable

Summary:
@public

The `WritableArray` and `WritableMap` interfaces currently require that nested arrays and maps also be writable. Nothing in our code actually relies on this, so we can relax this restriction and get useful properties.

For instance, it is now possible to construct a `JavaOnlyMap` (or array) that reuses `ReadableMap` and `ReadableArray` values by reference ( = structural sharing) instead of forcing a deep copy.

Reviewed By: kathryngray

Differential Revision: D16132580

fbshipit-source-id: 9f41189ebea2a82e775a7a4da8c357a5ce9c5b9d

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

@@ -101,8 +101,8 @@ public class JavaOnlyArray implements ReadableArray, WritableArray {
   }
 
   @Override
-  public JavaOnlyArray getArray(int index) {
-    return (JavaOnlyArray) mBackingList.get(index);
+  public ReadableArray getArray(int index) {
+    return (ReadableArray) mBackingList.get(index);
   }
 
   @Override
@@ -111,8 +111,8 @@ public class JavaOnlyArray implements ReadableArray, WritableArray {
   }
 
   @Override
-  public JavaOnlyMap getMap(int index) {
-    return (JavaOnlyMap) mBackingList.get(index);
+  public ReadableMap getMap(int index) {
+    return (ReadableMap) mBackingList.get(index);
   }
 
   @Override
@@ -161,12 +161,12 @@ public class JavaOnlyArray implements ReadableArray, WritableArray {
   }
 
   @Override
-  public void pushArray(@Nullable WritableArray array) {
+  public void pushArray(@Nullable ReadableArray array) {
     mBackingList.add(array);
   }
 
   @Override
-  public void pushMap(@Nullable WritableMap map) {
+  public void pushMap(@Nullable ReadableMap map) {
     mBackingList.add(map);
   }