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?

Right now, Application module contains a quite sloppy implementation of SDK version CSS classes.
Few things to note regarding the current implementation:

• Core keeps a needless cache of these css classes in application module top-level
• Core attempts to apply these classes to modal views during startup (there are no modals during app startup)
• Core attempts to initialize these classes using a function that is specifically for initializing a11y CSS classes

What is the new behavior?

This PR removes what's not needed and follows core API for applying system CSS classes.