fix: NS6/7 setMode fixes

[NathanaelA]

PR Checklist

• [X] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/tools/notes/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.
• [ ] 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?

• setMode crashes do to some changes in the NS7 runtimes
• setMode doesn't change the theme properly

What is the new behavior?

• setMode no longer crashes
• setMode changes the theme dynamically.

Notes

This has already been published as 2.3.4 - To fix a Internal Support Issue.