SystemUI: Allow showing data type with roaming indicator on MSIM.

  If a device sets a system property "ro.config.always_show_roaming"
  the roaming indicator will not replace the data type icon, but create
  a new indicator alongside signaling that the sim is roaming.

Change-Id: I60ac97572b9b50e4809dbd809a8e5a2d0763542e