Fixes #280 it looks like at some point this might have been changed from appCommon to Application and this reference missed.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages. (404)
• 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. #280
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/DevelopmentWorkflow.md#running-unit-tests. (404)
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/WritingUnitTests.md. N/A?

What is the current behavior?

#280

What is the new behavior?

Works as expected, calling Theme.setMode(Theme.Auto) resets to system's Dark Mode Setting.

Fixes #280.