MLS location layer requires additional memory and network.

Make this layer toggle an option that defaults to false.

Issue #921
Can you rename `ON_MAP_SHOW_MLS_PREF` to `SHOW_MLS_ON_MAP` and `ENABLE_SHOW_MLS_ON_MAP_PREF` to clarify that one is a flag to show MLS locations, and the other is a preference to toggle the availability of the flag.