Fine tune the UI of hamburger Menu

1. Change the order of root items.
2. Add item type in RootsAdapter to avoid to mix App items
   and Root items in ListView.

Fix: 111862616
Test: atest DocumentsUITests, atest DocumentsTest
Change-Id: Ic0647ba8719ac6d25dbda387a330ad77b56679d9
diff --git a/res/values/dimens.xml b/res/values/dimens.xml
index e5fe12a..414faba 100644
--- a/res/values/dimens.xml
+++ b/res/values/dimens.xml
@@ -20,6 +20,7 @@
     <dimen name="icon_size">40dp</dimen>
     <dimen name="root_icon_size">24dp</dimen>
     <dimen name="root_icon_margin">0dp</dimen>
+    <dimen name="root_spacer_padding">0dp</dimen>
     <dimen name="check_icon_size">30dp</dimen>
     <dimen name="list_item_thumbnail_size">40dp</dimen>
     <dimen name="grid_item_icon_size">30dp</dimen>