Fix Extension coupling to precise model class

Summary:
Since Extension (and Binder itself) can accept an Interface or abstract Class as the definition of the model, it's not correct to strictly prohibit exact type changes.

For example, if we want to bind a Drawable as a background, we should not limit the model to bind one exact Drawable type, but rather allow any Drawable.

And with the help of generics on Extension and Binder types we can be sure that `shouldUpdate` and `bind`/`unbind` methods will be called with the same model type for both current and new extension/binder.

Also, arguably, check for `binder` type is not needed too, since we use it (`newExtension.binder.getClass()`) as a key to retrieve the `currentExtension`, thus we'll have the same `binder` type guaranteed.

Reviewed By: pasqualeanatriello

Differential Revision: D23539921

fbshipit-source-id: a9016a2dbed89c5fc5e8d5dff2d99442c97ad63d

litho-rendercore/src/main/java/com/facebook/rendercore/RenderUnit.java

@@ -210,7 +210,7 @@ private static <MOUNT_CONTENT> void addExtension(
       // list.
       boolean found = false;
       for (int i = extensions.size() - 1; i >= 0; i--) {
-        if (extensions.get(i).equalTypes(extension)) {
+        if (extensions.get(i).binder.getClass() == extension.binder.getClass()) {
           extensions.remove(i);
           found = true;
           break;
@@ -419,23 +419,10 @@ public static <MODEL, CONTENT> Extension<MODEL, CONTENT> extension(
       return new Extension<>(model, binder);
     }
 
-    boolean equalTypes(Extension extension) {
-      return model.getClass() == extension.model.getClass()
-          && binder.getClass() == extension.binder.getClass();
-    }
-
     boolean shouldUpdate(
         final Extension<MODEL, CONTENT> prevExtension,
         final @Nullable Object currentLayoutData,
         final @Nullable Object nextLayoutData) {
-      if (!equalTypes(prevExtension)) {
-        throw new IllegalArgumentException(
-            "The types to operate on do not match!\nNew model: "
-                + model
-                + ", old model: "
-                + prevExtension.model);
-      }
-
       return binder.shouldUpdate(prevExtension.model, model, currentLayoutData, nextLayoutData);
     }