dock:: fix dock cannot be shown after dde-session-daemon is restarted

tower:https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/1f07dd7cb0724948bad734ae6ad98274/
Change-Id: Ib9f4b41a684836c31fa9a2778f6f2b31c090d4ea