Create a copy of the original list instead of a new view

unmodifiableList is backed up by the original list and is not suitable
to passing to different thead.

Bug: 162518472
Change-Id: I71774ced89d64acc7176269e107cfcf47627f7c4

src/com/android/launcher3/model/BgDataModel.java

@@ -422,7 +422,7 @@ public class BgDataModel {
 
         @Override
         public FixedContainerItems clone() {
-            return new FixedContainerItems(containerId, Collections.unmodifiableList(items));
+            return new FixedContainerItems(containerId, new ArrayList<>(items));
         }
     }