Summary:
This allows the `NavigationCard` style to be extended, the default `backgroundColor` style of the `NavigationCard` is not applicable to all apps.
Fixes issue #8116. PR #8115 was already made for this but didn't pass CI checks.
**Test plan**
When rendering a NavigationCardStack, add
```
const { CardStack } = NavigationExperimental;
...
<CardStack cardStyle={{ backgroundColor: 'purple' }} />
```
to see that the `NavigationCard` background has changed to purple.
Closes https://github.com/facebook/react-native/pull/8471
Differential Revision: D3508849
Pulled By: javache
fbshipit-source-id: 65703cccf3d01def9f2060256f0ff564bbdc47d3
aa0efd2e64