Caching last predictions and loading it with model

Adding support for persisting itemInfos on disk. This uses
a separate xml file. Unlike prefs, it does not keep the items
in memory and is just a wraper over reading/writing a file.

Bug: 160748731
Change-Id: Iaccab9928ab8f30127fb3c2d630ca8ca83f0bd05

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

@@ -424,6 +424,14 @@ public class BgDataModel {
         public FixedContainerItems clone() {
             return new FixedContainerItems(containerId, new ArrayList<>(items));
         }
+
+        public void setItems(List<ItemInfo> newItems) {
+            items.clear();
+            newItems.forEach(item -> {
+                item.container = containerId;
+                items.add(item);
+            });
+        }
     }