Coverage overlay zoom

This moves the coverage overlay into its own class which, for the moment, extends the TilesOverlay.  Once it is changed to be purely vector based, it no longer need do so, but there should not be changes necessary to the MapActivity at that point as it sees the coverage map as a simple Overlay.

When asked to draw zoom levels greater than those provided by the coverage map (currently level 13), this will now determine a scaling factor, grab the lower zoom level image, and do a simple scale to draw the overlay.

Once the vectorization is at a happy point, it should be able to be swapped into the mCoverageTileLooper with minimal fuss.