Throttle the folder loader updates

With large batches of operations, the state of the folder
changes a lot.  This causes a lot of update in the ui to update
the unread count.  Instead delay the updates of the loader for
500ms from the last call to onLoadInBackground()

Change-Id: I7e7872e5888c74965fd9ca1a86c668cee5a79387
diff --git a/res/values/constants.xml b/res/values/constants.xml
index 562f681..b882004 100644
--- a/res/values/constants.xml
+++ b/res/values/constants.xml
@@ -44,4 +44,7 @@
     <integer name="swipeScrollSlop">2</integer>
    <!-- <integer name="widget_refresh_delay_ms">4000</integer>-->
     <integer name="widget_folder_refresh_delay_ms">500</integer>
+    <!-- We don't want the unread count to flicker too much in the action bar, when a lot of
+    activity.  Throttle loader updates. -->
+    <integer name="folder_item_refresh_delay_ms">500</integer>
 </resources>