Deliver OnMapReady only when the map is fully initialized

[LukasPaczos]

Race condition patched up in the #13203 introduced/exposed another bug - instead of marking map as initialized when MapboxMap object is created, we should do that at the end of the initialization block, to not deliver the onMapReady prematurely.

This can lead to crashes like

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.mapbox.mapboxsdk.maps.MapGestureDetector.addOnMapClickListener(com.mapbox.mapboxsdk.maps.MapboxMap$OnMapClickListener)' on a null object reference
    at com.mapbox.mapboxsdk.maps.MapView$GesturesManagerInteractionListener.onAddMapClickListener(MapView.java:1420)
    at com.mapbox.mapboxsdk.maps.MapboxMap.addOnMapClickListener(MapboxMap.java:2133)
    at com.mapbox.mapboxsdk.testapp.activity.maplayout.DoubleMapActivity$DoubleMapFragment.lambda$onViewCreated$1$DoubleMapActivity$DoubleMapFragment(DoubleMapActivity.java:100)
    at com.mapbox.mapboxsdk.testapp.activity.maplayout.DoubleMapActivity$DoubleMapFragment$$Lambda$0.onMapReady(Unknown Source:2)
    at com.mapbox.mapboxsdk.maps.MapView$MapCallback.onMapReady(MapView.java:1602)
    at com.mapbox.mapboxsdk.maps.MapView$MapCallback.initialise(MapView.java:1591)
    at com.mapbox.mapboxsdk.maps.MapView.initialiseMap(MapView.java:181)
    at com.mapbox.mapboxsdk.maps.MapView.access$800(MapView.java:76)
    at com.mapbox.mapboxsdk.maps.MapView$6.run(MapView.java:336)