Remember the chosen custom background when set by url

[pgiraud]

Here's a quick workaround for #4162.
@bhousel I'm not sure if it's the best way to fix the issue but it seem to work.

[bhousel]

Thanks so much @pgiraud - this is pretty good!

Your quick fix worked because it was creating a Custom imagery layer and setting the localStorage template - then the real Custom layer would get created later when the Background ui was added and that template would get picked.

I added a commit after to cleanup the ownership of these objects. Now all the layers, including Custom and None are 'owned' by rendererBackground(), and the uiBackground() does not create a Custom layer, it just finds the existing one by id. The new code is a little easier to reason about.