PlaceListNavigationTemplate doen't show header.

Hello colleagues,

According to [Android for Cars App Library design guidelines 1.3-beta](https://developer.android.com/static/training/cars/Android%20for%20Cars%20App%20Library%20design%20guidelines.pdf) @ page 65: [PlaceListNavigationTemplate](https://developer.android.com/reference/androidx/car/app/navigation/model/PlaceListNavigationTemplate) should have a header element.
Please take a look sample code, [PlaceListNavigationTemplate](https://github.com/android/car-samples/blob/main/car_app_library/showcase/common/src/main/java/androidx/car/app/sample/showcase/common/navigation/PlaceListNavigationTemplateDemoScreen.java) @ line 89:
` .setHeader(header)`
But, it doesn't appear on screen:

Could you please help us solve this issue?

Start with showcase/common/src/main/java/androidx/car/app/sample/showcase/common/navigation/PlaceListNavigationTemplateDemoScreen.java at line 89, where the header is set, and compare the rendered screen with page 65 of the linked design guidelines. Reproduce the sample and trace why the header is not displayed; done means the PlaceListNavigationTemplate screen shows its header as expected.

Written by the indexing model from the issue text.