PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/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.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/DevelopmentWorkflow.md#running-unit-tests-application.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/tools/notes/WritingUnitTests.md.

What is the current behavior?

On iOS, the isLayoutValid getter can be true when checked in a layoutChanged event listener.
This causes platform inconsistency and reports a valid layout before computations are over.

What is the new behavior?

Updated the getter to reflect the android approach since the core layout system for iOS is inspired from it and added an automated test to cover the case explained.
This is helpful for plugins to make use of that flag and have identical results for both platforms.
Additionally, causing the flag to be truthy at a later point makes it wait for layoutIfNeeded call, thus making it usable for tests that didn't pass till now.