fix(ios): Add/remove shadow for reusable views

[CatchABus]

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• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

When it comes to reusable views, it seems that view box shadow is not removed when view gets removed from view tree.
That is because shadow layer belongs to parent and we rely on disposeNativeView to cleanup shadows but that doesn't seem to be the right approach.

What is the new behavior?

Add/remove shadows along with view, regardless of being reusable or not. It's also a good replacement for my previous PR.