dlx: system.prop: Enable MTP by default

This fixes a derp I made in

  commit 7580945763efd7b5bc7d869cc9df95d192d61021
  Author: Christopher R. Palmer <crpalmer@gmail.com>
  Date:   Tue Dec 3 20:24:25 2013 -0500

    dlx: system.prop: Remove duplicated entries

Change-Id: I64e37f70bab00b11b80130eb4607087142626be6