Identifying exit numbers in roundabouts

Dieses Issue hat noch niemand übernommen.

**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"](https://github.com/mapbox/mapbox-navigation-android/blob/bd097da71d4240b064af9a58614f3435cf803106/libnavui-maneuver/src/main/java/com/mapbox/navigation/ui/maneuver/model/PrimaryManeuver.kt#L33)

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

Beginne mit den Daten zu Kreisverkehrsmanövern aus der Directions API und der im Issue verlinkten Definition von PrimaryManeuver.kt. Verfolge, wie die Manöverfelder die navigation-ui Maneuver API erreichen, und stelle dann die Ausfahrtsnummer des Kreisverkehrs mithilfe der Definition von Android Auto Maneuver.setRoundaboutExitNumber bereit; abgeschlossen ist die Aufgabe, wenn die angeforderte Ausfahrtsnummer für diese Route verfügbar ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.