Add scaffolds for performance tests of DocumentsUI

Bug: 27370274
Change-Id: I14dea1b85cd84c8bb3c0eee27b2954108bfa4f8b
diff --git a/src/com/android/documentsui/dirlist/Model.java b/src/com/android/documentsui/dirlist/Model.java
index 5e55e1a..8170e2a 100644
--- a/src/com/android/documentsui/dirlist/Model.java
+++ b/src/com/android/documentsui/dirlist/Model.java
@@ -64,6 +64,7 @@
 
     @Nullable String info;
     @Nullable String error;
+    @Nullable DocumentInfo doc;
 
     /**
      * Generates a Model ID for a cursor entry that refers to a document. The Model ID is a unique
@@ -111,6 +112,7 @@
             mPositions.clear();
             info = null;
             error = null;
+            doc = null;
             mIsLoading = false;
             notifyUpdateListeners();
             return;
@@ -125,6 +127,7 @@
         mCursor = result.cursor;
         mCursorCount = mCursor.getCount();
         mSortOrder = result.sortOrder;
+        doc = result.doc;
 
         updateModelData();