Make layer and source ownership consistent

[ivovandongen]

Like iOS in #6254, we need some refactoring to make the layer/source ownership more convenient for developers. Atm, after adding a newly created layer/source to the map, the original java reference becomes unusable. To facilitate this a bit, we throw a Java exception to report misuse of the api to the developer. But this is not well understood.

Instead, we can also keep a raw pointer to the layer/source after transferring ownership and make sure the reference to the map is set.

ToDo:

• Get rid of explicit checks for validity
• Refactor core object transfer so that the native map reference is set when adding the layer/source to the map (needed for CustomLayer::update)
• Ensure that adding a layer/source to the map that was already added throws a nice Java exception
• Add JavaDoc:
  • MapboxMap#addLayer
  • MapboxMap#removeLayer
  • MapboxMap#addSource
  • MapboxMap#removeSource

[jfirebaugh]

#6959 and the discussion there is also relevant.

[boundsj]

I proposed #7014 for #6959. If there is interest in using in for Android see #7014 (comment)

[ivovandongen]

@jfirebaugh @boundsj Thanks, I missed that ticket completely.

@boundsj If you could get your PR merged, I'll make the necessary changes to this one to take advantage of it for Android.

[ivovandongen]

@jfirebaugh @boundsj I've created a separate ticket (#7023) for the ownership transfer on remove so these changed are not held up by that.

cc @tobrun @zugaldia