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

src/com/android/launcher3/ButtonDropTarget.java

@@ -264,6 +264,10 @@ public abstract class ButtonDropTarget extends TextView
      */
     @Override
     public void onDrop(final DragObject d, final DragOptions options) {
+        if (options.isFlingToDelete) {
+            // FlingAnimation handles the animation and then calls completeDrop().
+            return;
+        }
         final DragLayer dragLayer = mLauncher.getDragLayer();
         final Rect from = new Rect();
         dragLayer.getViewRectRelativeToSelf(d.dragView, from);