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/BaseActivity.java

@@ -17,7 +17,6 @@
 package com.android.launcher3;
 
 import static com.android.launcher3.util.SystemUiController.UI_STATE_OVERVIEW;
-
 import static java.lang.annotation.RetentionPolicy.SOURCE;
 
 import android.app.Activity;