Get geoJSON parameter from Mapbox Layer - android

Question

I followed the Mapbox Android tutorial to draw Polygons with geoJSON: https://www.mapbox.com/android-docs/maps/overview/data-driven-styling/#geojson

It works, but I thought that I could not call "the Polygon's id" in geoJSON from a layer.

I want to call this for detecting that I stand on the Polygons with smartphone location.

How can I call "the Polygon's id" in geoJSON from a layer?

Here are my codes(I wrote in Kotlin, and I know Android Java):

// draw polygon from geoJSON
private fun drawPolygon(map: MapboxMap) {
    val source = GeoJsonSource("sourceId", GEOJSON_STRING) //following
    map.addSource(source)
    val layer = FillLayer("layerId", "sourceId")
    map.addLayer(layer)

    findPolygonId(layer)
}

// find polygon id from layer
findPolygonId(layer: Layer) {
    val id = layer.find(geoJSON.id) <--- unknown code
}

GEOJSON_STRING is following:

{
  "type": "FeatureCollection",
  "features": [
    {
      "type": "Feature",
      "id": 1, <-----------------I want call this!
      "properties": {
        "stroke": "#000000",
        "stroke-width": 3.3,
        "stroke-opacity": 0,
        "fill": "#000000",
        "fill-opacity": 1
      },
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
            [
              Lng1,
              Lat1
            ],
            [
              Lng2,
              Lat1
            ],
            [
              Lng2,
              Lat3
            ],
            [
              Lng1,
              Lat3
            ],
            [
              Lng1,
              Lat1
            ]
          ]
        ]
      }
    },
    {
      "type": "Feature",
      "id": 2, <-----------------I want call this!
      "properties": {
        "stroke": "#000000",
        "stroke-width": 3.3,
        "stroke-opacity": 0,
        "fill": "#000000",
        "fill-opacity": 1
      },
      "geometry": {
        "type": "Polygon",
        "coordinates": [
          [
          [
            [
              Lng4,
              Lat4
            ],
            [
              Lng5,
              Lat4
            ],
            [
              Lng5,
              Lat6
            ],
            [
              Lng4,
              Lat6
            ],
            [
              Lng4,
              Lat4
            ]
          ]
        ]
      }
    }
  ]
}