在Xamarin.Android中显示地图，从未调用过OnMapReady

<uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />

<meta-data
    android:name="com.google.android.geo.API_KEY"
    android:value="YOUR_API_KEY" />

<com.google.android.gms.maps.MapView
    android:id="@+id/mapView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

using Android.App;
using Android.OS;
using Android.Support.V7.App;
using Android.Widget;
using Com.Google.Android.Gms.Maps;
using Com.Google.Android.Gms.Maps.Model;

[Activity(Label = "MapActivity")]
public class MapActivity : AppCompatActivity, IOnMapReadyCallback
{
    private GoogleMap googleMap;
    private MapView mapView;

    protected override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);
        SetContentView(Resource.Layout.activity_map);

        mapView = FindViewById<MapView>(Resource.Id.mapView);
        mapView.OnCreate(savedInstanceState);
        mapView.GetMapAsync(this);
    }

    public void OnMapReady(GoogleMap map)
    {
        googleMap = map;

        // 在地图上添加标记
        LatLng location = new LatLng(37.7749, -122.4194);
        MarkerOptions markerOptions = new MarkerOptions()
            .SetPosition(location)
            .SetTitle("San Francisco");
        googleMap.AddMarker(markerOptions);

        // 移动地图视图到指定位置
        CameraUpdate cameraUpdate = CameraUpdateFactory.NewLatLngZoom(location, 12);
        googleMap.MoveCamera(cameraUpdate);
    }

    protected override void OnResume()
    {
        base.OnResume();
        mapView.OnResume();
    }

    protected override void OnPause()
    {
        base.OnPause();
        mapView.OnPause();
    }

    protected override void OnDestroy()
    {
        base.OnDestroy();
        mapView.OnDestroy();
    }

    public override void OnLowMemory()
    {
        base.OnLowMemory();
        mapView.OnLowMemory();
    }
}