How to make flutter google map prompt "Allow all the time" when asking permission the first time?

Question

When google maps loads, location permission only asks to "Allow only while using the app". I want it to ask permission to allow all the time so that the location can be queried in the background by workmanger. How do I make it prompt as "Allow all the time"? Full code here. Thanks for reading. This should be pretty easy to figure out, sometimes I can get it to work on another github example here but somehow it breaks after I make changes.

import 'dart:math';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'package:flutter/material.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'package:location/location.dart' as loco;
import 'package:ungshowlocation/models/marker_collect_model.dart';
import 'package:ungshowlocation/widget/add_location.dart';
import 'package:ungshowlocation/widget/detail_marker.dart';
import 'package:ungshowlocation/widget/my_service.dart';

class ShowMap extends StatefulWidget {
  final double lat;
  final double lng;
  ShowMap({Key key, this.lat, this.lng}) : super(key: key);

  @override
  _ShowMapState createState() => _ShowMapState();
}

class _ShowMapState extends State<ShowMap> {
  // Field
  double lat, lng;
  BitmapDescriptor policeIcon;
  List<Marker> list = List();
  List<String> listDocuments = List();
  final Set<Polyline> _polyline = {};
  GoogleMapController controller;
  List<LatLng> latlngSegment1 = List();
  List<LatLng> latlngSegment2 = List();
  static LatLng _lat1 = LatLng(45.19, -121.59);
  static LatLng _lat2 = LatLng(45.30, -122.20);
  static LatLng _lat3 = LatLng(45.11, -122.61);
  static LatLng _lat4 = LatLng(45.42, -122.62);
  static LatLng _lat5 = LatLng(45.34, -122.32);
  static LatLng _lat6 = LatLng(45.21, -122.2);
  bool _myLocationButtonEnabled = true;
  bool _myLocationEnabled = true;

  // Method

  @override
  void initState() {
    super.initState();
    // findLatLng();

    readDataFromFirebase();

    setState(() {
      lat = widget.lat;
      lng = widget.lng;
      latlngSegment1.add(_lat1);
      latlngSegment1.add(_lat2);
      latlngSegment1.add(_lat3);
      latlngSegment1.add(_lat4);

      //line segment 2
      latlngSegment2.add(_lat4);
      latlngSegment2.add(_lat5);
      latlngSegment2.add(_lat6);
      latlngSegment2.add(_lat1);
    });
  }

    Future<Null> readDataFromFirebase() async {
    print('###############readDataFromFirebase Work####################');

    Firestore firestore = Firestore.instance;
    CollectionReference collectionReference =
    firestore.collection('usertest');
    await collectionReference.snapshots().listen((event) {
      List<DocumentSnapshot> snapshots = event.documents;
      for (var map in snapshots) {
        MarkerCollectModel model = MarkerCollectModel.fromMap(map.data);
        String nameDocument = map.documentID;
        listDocuments.add(nameDocument);
        print('Name ==>> ${model.name}');
        Marker marker = createMarker(model, nameDocument);
        setState(() {
          list.add(marker);
          print('myMarkers set lenght ==>> ${myMarkers().length}');
        });
      }
    });
  }

  Marker createMarker(MarkerCollectModel markerCollectModel, String nameDocument) {
    Marker marker;
    Random random = Random();
    int i = random.nextInt(100);
    String idString = 'id$i';

    marker = Marker(
      markerId: MarkerId(idString),
      position: LatLng(markerCollectModel.lat, markerCollectModel.lng),
      infoWindow: InfoWindow(
          title: markerCollectModel.name, snippet: markerCollectModel.detail),
      onTap: () {
        print('You Tab Name ==>> ${markerCollectModel.name}');

        MaterialPageRoute route = MaterialPageRoute(
          builder: (context) => DetailMarker(
            model: markerCollectModel,nameDocument: nameDocument,
          ),
        );
        Navigator.push(context, route);
      },
    );
    return marker;
  }

  Future<void> findLatLng() async {
    loco.LocationData locationData = await findLocation();
    setState(() {
      lat = locationData.latitude;
      lng = locationData.longitude;
      print('lat =>>> $lat, lng ===>> $lng');
    });
  }

  Future<loco.LocationData> findLocation() async {
    var location = loco.Location();
    try {
      return await location.getLocation();
    } catch (e) {
      print('e location = ${e.toString()}');
      return null;
    }
  }

  Set<Marker> myMarkers() {
    list.add(localMarker());
    return list.toSet();
  }

  Marker localMarker() {
    return Marker(
      infoWindow: InfoWindow(
      ),
      markerId: MarkerId('myLocotion'),
    );
  }

  Widget showMap() {
    print('latlng on showmap ===>>> $lat, $lng');
    LatLng centerLatLng = LatLng(lat, lng);
    CameraPosition cameraPosition =
    CameraPosition(target: centerLatLng, zoom: 16.0);

    return Stack(
      children: <Widget>[
        GoogleMap(
          initialCameraPosition: cameraPosition,
          mapType: MapType.normal,
          markers: myMarkers(),
          polylines: _polyline,
          myLocationEnabled: _myLocationEnabled,
          myLocationButtonEnabled: _myLocationButtonEnabled,
          onMapCreated: _onMapCreated,
        ),
        // addButton(),
      ],
    );
  }

  Widget addButton() {
    return Row(
      mainAxisAlignment: MainAxisAlignment.end,
      children: <Widget>[
        Column(
          mainAxisAlignment: MainAxisAlignment.end,
          children: <Widget>[
            Container(
              margin: EdgeInsets.only(
                right: 40.0,
                bottom: 40.0,
              ),
              child: FloatingActionButton(
                onPressed: () {
                  MaterialPageRoute route = MaterialPageRoute(
                      builder: (value) => MyService(
                        currentWidget: PageWidget(
                        ),
                      ));
                  Navigator.of(context)
                      .pushAndRemoveUntil(route, (value) => false);
                },
                child: Icon(
                  Icons.add_circle,
                  size: 36.0,
                ),
              ),
            ),
          ],
        ),
      ],
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: lat == null
          ? Center(
        child: CircularProgressIndicator(),
      )
          : showMap(),
    );
  }
  void _onMapCreated(GoogleMapController controllerParam) {
    setState(() {
      controller = controllerParam;
      _polyline.add(Polyline(
        polylineId: PolylineId('line1'),
        visible: true,
        //latlng is List<LatLng>
        points: latlngSegment1,
        width: 2,
        color: Colors.blue,
      ));

      //different sections of polyline can have different colors
      _polyline.add(Polyline(
        polylineId: PolylineId('line2'),
        visible: true,
        //latlng is List<LatLng>
        points: latlngSegment2,
        width: 2,
        color: Colors.red,
      ));
    });
  }
}