MapView is disposed way too long with a lot of Graphic in certain conditions

[dev4567]

# Context

We use `GraphicsOverlay` with a lot of graphic items (~10k). 

# Issue

When the `MapView` is destroyed right after the graphic items are added (e.g. within 5-10 secs) the `MapView` disposes for 2-5 seconds, which causes ANR . Instead if we wait for 5-10 secs right after adding the graphics and only then try to destroy `MapView`, dispose takes milliseconds and everything works fine.

# Details

As far as we understand 

this.getGraphicsOverlays().clear();

takes too much time if being called right after graphics are added to the `GraphicsOverlay`

Here is the callstack: one could see `clear` takses mostly all the time of the `destroy` function.

The issue is also seen in the LogCat (here are the logs for the Activity onStop/onStart lifecycle): TextureView disposing takes 15 secs

I want to emphasise once more what is weird: if we wait for 5-10 secs right after adding the items and before destroying the view, disposing works normally.

[dev4567]

important note: the problems exists only if

renderingMode = GraphicsRenderingMode.Dynamic

with `Static` rendering mode dispose works good

[dev4567]

We also see a very similar issue with the `FeatureCollectionLayer`, but now the disposable choke point is `setMap(null)`