fix: trigger css update when setting root classes

[rigor789]

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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?

In some scenarios setting the initial root css classes doesn't trigger a style update.

What is the new behavior?

We force update styling whenever setting root css classes.

fixes #10318

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