Add undo snackbar for deleting items

- Add methods to ModelWriter to prepareForUndoDelete, then
  enqueueDeleteRunnable, followed by commitDelete or abortDelete.
- Add Snackbar floating view
- Show Undo snackbar when dropping or flinging to delete target; if the
  undo action is clicked, we abort the delete, otherwise we commit it.

Bug: 24238108
Change-Id: I9997235e1f8525cbb8b1fa2338099609e7358426
diff --git a/res/values/dimens.xml b/res/values/dimens.xml
index 58fce34..e7bd64c 100644
--- a/res/values/dimens.xml
+++ b/res/values/dimens.xml
@@ -225,4 +225,10 @@
 <!-- Overview -->
     <dimen name="options_menu_icon_size">24dp</dimen>
     <dimen name="options_menu_thumb_size">32dp</dimen>
+
+<!-- Snackbar -->
+    <dimen name="snackbar_height">48dp</dimen>
+    <dimen name="snackbar_content_height">32dp</dimen>
+    <dimen name="snackbar_padding">8dp</dimen>
+    <dimen name="snackbar_margin">16dp</dimen>
 </resources>