demo: allow user to use device location as start point

[reinterpretcat]

so far, there are two options:

• use hardcoded one (Moscow, Red Square). It is default as apk will be shipped with map data for this region.
• use geocoding service to resolve geocoordinate by name

The third one can be additional button with action "use device location". Having reverse geocoding functionality for it is also nice.

Assumed that this class provides access to location data: https://docs.unity3d.com/ScriptReference/LocationService.Start.html

[reinterpretcat]

Done