Update offline navigation example.

This Pr is to update offline navigation example with the following process.

A button to download an offline region.
Show the offline region boundary around user's current location.
After the offline region is downloaded, allow user to choose destination and request directions.
After the direction is requested, ask the user to turn off network access to start navigation.
A button to start simulated navigation in this offline region.
The example downloads a region around the current user location.

mapbox / mapbox-navigation-ios-examples