Odometer feature (1st cut)
Odometer, take 2
Add display of zoom level into main activity
Make odometer estimate change more often when walking
Implement drawing of map overlays for displaying coverage data

This removes the need to render a whole set of tiles offline and upload on to
the phone.  The replacement is one relatively small 'overlay' file per network.

Squashed commit of the following:
    Merge branch 'master' into overlay
    Don't anti alias the stroke paints for the square boxes
    Add short vane rendering
    Prevent mis-parse of the file running for a long time
    Merge branch 'master' into overlay
    Add display of zoom level into main activity
    Rework map menu to use overlays for everything
    Fix map download behind overlays
    Render captions on towers
    Implement rendering for null coverage mini-tiles
    Implement to-do maps from overlay information
    Checkpoint overlay work
    Part-finished overlay implementation