Add PathOverlay to the map, to draw the GPS track

Also, use a consistent color for the GPS dot and track.

Note that post OSMDroid-4.2, PathOverlay is deprecated, and OSMDroid
recommends using Polyline in the OSMBonusPack:

https://github.com/osmdroid/osmdroid/issues/40
Merge pull request #727 from keverets/local-track

Add PathOverlay to the map to draw the GPS track