With no wifi, switch to low-resolution tiles.

- Swap in an empty base tile layer when no wi-fi (and show a Toast), and show a low-resolution coverage map instead. 
- Instead of getting many levels of tiles for each zoom (typically I see 10 levels on disk), just get 2 levels. 
- Reduced the lowest zoom level of the map (to 30 km x 30 km approx on Nexus 5 screen) to prevent pulling in unnecessary data for areas the user isn't stumbling. 
 
ASIDE: Increased the max zoom to level 20, which is handy for separating the stumbling dots when walking.

TODO
- track the delta for the tile dir when not on wi-fi
- add a switch on the developer screen for tile source: default/lowres-only/high-res only
Magic numbers again.  Pull out 13, 13 as MIN and MAX zoom level constants.