fix: modal beforeClosed and afterClosed consistency between platforms

[edusperoni]

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/angular/blob/master/DevelopmentWorkflow.md#running-the-tests
• Tests for the changes are included.

What is the current behavior?

Currently when a modal closes the events fire quickly, often before the modal has fully closed. As a result, navigations done directly on the afterClosed callback won't work, needing a 0ms delay on iOS and >100ms on Android.

What is the new behavior?

We delay the afterClosed callback until we're sure the modal has fully closed