Identifying exit numbers in roundabouts

Mapbox Navigation SDK version: 2.0.0-rc.2

https://hey.mapbox.com/directions-debug/#route=6.124388228364182,45.9055553186453;6.126108200540244,45.90652815414734&server=https://api.mapbox.com&profile=mapbox/driving-traffic&style=empty&queryparams=steps%3Dtrue%26overview%3Dfull%26geometries%3Dgeojson%26roundabout_exits%3Dtrue%26voice_units%3Dimperial%26voice_instructions%3Dtrue&token=pk.eyJ1Ijoic3ZjLW9rdGEtbWFwYm94LXN0YWZmLWFjY2VzcyIsImEiOiJja2d0bHMxbDcwNHI0MnRxbHBwNTBoeHN0In0.X5Gn0apaqXAXDkZ1sSDLfA

Directions API provides an exit number

              "maneuver": {
                "type": "roundabout",
                "exit": 3,
                "instruction": "Enter the roundabout and take the 3rd exit onto Avenue de Brogny/D 1201.",
                "modifier": "slight right",
                "bearing_after": 123,
                "bearing_before": 94,
                "location": [
                  6.125695,
                  45.90563
                ]
              },

But I can't find this exit number from the navigation-ui Maneuver api. It seems like it should be near the "degrees"

Here's the definition we're looking to satisfy for Android Auto from their Manevuer.setRoundaboutExitNumber

For example, if the driver is joining a counter-clockwise roundabout with 4 exits,
then the exit to the right would be exit #1, the one straight ahead would be exit #2,
the one to the left would be exit #3 and the one used by the driver to join the
roundabout would be exit #4.

cc: @cafesilencio @abhishek1508 @Guardiola31337

Start with the roundabout maneuver data from the Directions API and the PrimaryManeuver.kt definition linked in the issue. Trace how maneuver fields reach the navigation-ui Maneuver API, then expose the roundabout exit number using the Android Auto Maneuver.setRoundaboutExitNumber definition; done means the requested exit number is available for this route.