Fix navigation in the TV demo app

[MGaetan89]

Pull request

Description

There was some issues when navigating in the TV demo app:

• When in "Lists", it was hard to give the focus back to the top navigation to navigate to an other section
• When in "Lists", pressing back would show the "Examples" section, but the top navigation would still show "Lists" as selected

Changes made

• Observe navigation changes on root screens to update the top navigation accordingly
• Migrate Compose previews to PillarboxTheme instead of using the default MaterialTheme
• Observe press on the up arrow to give focus to the top navigation when needed

Checklist

• Your branch has been rebased onto the main branch.
• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).
• All pull request status checks pass.