mako: Move XTRA_SERVER entries to overlays

* XTRA_SERVERs are important, right? Like to get the almanac data
  necessary for aGPS. Without the XTRA download, the chip needs to
  collect sufficient navigation messages from the birds to compute
  where they are in order to make sense of the ranging signals.
* Well, it seems that O doesn't like reading these entries from the
  gps.conf file.

  When in gps.conf:
  GpsXtraDownloader: No XTRA servers were specified in the GPS configuration

  When in overlay:
  <that noise doesn't exist>

* This seems to, finally, return GPS fix performance to what we had
  in N.

Change-Id: I70679835ec5dea053c5aa3750acee628906d6390