Nullify taskbar hotseatView tag when the cell is empty

This ensures that a removed view becomes invisible.

Test: turn off hotseat suggestions, drag an item out
and verify it's removed from taskbar as well
Bug: 179886115
Bug: 171917176

Change-Id: Iec509c2f0c420cee6f86feb293419bb2249a8dc4

quickstep/src/com/android/launcher3/taskbar/TaskbarView.java

@@ -192,6 +192,7 @@ public class TaskbarView extends LinearLayout implements FolderIcon.FolderIconPa
             } else {
                 hotseatView.setOnClickListener(null);
                 hotseatView.setOnLongClickListener(null);
+                hotseatView.setTag(null);
             }
             updateHotseatItemVisibility(hotseatView);
         }