fix(system-theme): Make sure setting Theme to Auto applies the changes

[bundyo]

Fix a border-radius error in RDF

PR Checklist

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

What is the current behavior?

If the device is in dark mode, setting forced theme to light/dark and back to auto leaves the app in light mode.

What is the new behavior?

The app is returned to the current device mode.

Fixes/Implements/Closes #265.