- Custom annotations would sometimes not get rendered on the screen.
  Haven't nailed down the exact problem, but it was some sort of
  interleaving of operations from adding children and computing custom
  marker views. Moving the marker view stuff to when the map finishes
  loading resolves this.
- The adapter for the react native marker views now handles the caching
  behaviour correctly and re-uses views when they are available.
- Make sure each layout actually gets the height and width of the react
  native views from the correct shadow nodes so that they don't all have
  the same measurements by accident.
- Update locations of annotations when they are changed.
- Split annotation views into a map. The manager is a singleton and all
  the existing map views go through it so the children need to be kep
  separately.
- Safeguard against relayout being called after onDrop(), resulting in a
  null pointer exception.