Add cardStyle prop to NavigationCardStack.

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

Libraries/CustomComponents/NavigationExperimental/NavigationCardStack.js

@@ -64,6 +64,7 @@ type Props = {
   onNavigateBack?: Function,
   renderOverlay: ?NavigationSceneRenderer,
   renderScene: NavigationSceneRenderer,
+  cardStyle?: any,
   style: any,
 };
 
@@ -96,6 +97,7 @@ class NavigationCardStack extends React.Component<DefaultProps, Props, void> {
     onNavigateBack: PropTypes.func,
     renderOverlay: PropTypes.func,
     renderScene: PropTypes.func.isRequired,
+    cardStyle: View.propTypes.style,
   };
 
   static defaultProps: DefaultProps = {
@@ -192,7 +194,7 @@ class NavigationCardStack extends React.Component<DefaultProps, Props, void> {
         key={'card_' + props.scene.key}
         panHandlers={panHandlers}
         renderScene={this.props.renderScene}
-        style={style}
+        style={[style, this.props.cardStyle]}
       />
     );
   }