PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
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• All existing tests are passing: https://github.com/NativeScript/nativescript-angular/blob/master/DevelopmentWorkflow.md#running-the-tests
• Tests for the changes are included.

What is the current behavior?

Modal window in iOS 13 is transparent.

What is the new behavior?

Modal window in iOS 13 is not transparent. In iOS 13 the style of the modals has changed and we need ot explicitly set the backgroundColor of the first element that is passed to showModal

Fixes/Implements/Closes #[Issue Number].
Resolves #2012