only backup widgets assets once

Bug: 10778984
Change-Id: I6736f8b699c7fc5669f966a8b25819ea380480ab
diff --git a/src/com/android/launcher3/LauncherBackupAgent.java b/src/com/android/launcher3/LauncherBackupAgent.java
index 7cffe7b..cb0dd49 100644
--- a/src/com/android/launcher3/LauncherBackupAgent.java
+++ b/src/com/android/launcher3/LauncherBackupAgent.java
@@ -567,6 +567,7 @@
                         previewLoader.setPreviewSize(spanX * profile.cellWidthPx,
                                 spanY * profile.cellHeightPx, widgetSpacingLayout);
                         byte[] blob = packWidget(dpi, previewLoader, iconCache, provider);
+                        keys.add(key);
                         writeRowToBackup(key, blob, out, data);
 
                     } else {