Adding example of onStyleImageMissing listener usage

[osana]

This PR adds an example for handling missing image in Style.

If an icon-image cannot be found in a map Style, a custom image could be provided via
onStyleImageMissing listener

This example is based on @captainbarbosa's suggestion from
mapbox/ios-sdk-examples#289

[langsmith]

Ok, I've refactored this to mainly add more logic. This example is based around the idea of using onStyleImageMissing() to set an appropriate user profile photo at runtime. The profile photo SymbolLayer uses iconImage(get(PROFILE_NAME)), and then the switch/case below uses the id to set the right icon image

@Override
      public void onStyleImageMissing(@NonNull String id) {
        switch (id) {
          case CARLOS:
            addImage(id, R.drawable.carlos);
            break;
          case ANTONY:
            addImage(id, R.drawable.antony);
            break;
          case MARIA:
            addImage(id, R.drawable.maria);
            break;
          case LUCIANA:
            addImage(id, R.drawable.luciana);
            break;
          default:
            addImage(id, R.drawable.carlos);
            break;
        }
      }

private void addImage(String id, int drawableImage) {
    Style style = mapboxMap.getStyle();
    if (style != null) {
      style.addImageAsync(id, BitmapUtils.getBitmapFromDrawable(
        getResources().getDrawable(drawableImage)));
    }
  }

[tobrun]

looks good