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
L324 and L333,334 are racy.

cm.getActiveNetworkInfo().isConnectedOrConnecting() just indicate that any kind of network connection including cellular is established.  L333,334 checks that the active network is Wifi.  If the network connection is flapping between wifi and cellular, this can still set the high res map on.

Let's change isConnectedOrConnecting() to isConnected() to minimize the impact of network flapping, and move the info check on L333 to immediately before L324.