Fix regression for shadow in header on Android

packages/react-navigation/src/views/Header/Header.js

@@ -503,7 +503,9 @@ class Header extends React.PureComponent {
       <Animated.View
         style={[
           this.props.layoutInterpolator(this.props),
-          { backgroundColor: DEFAULT_BACKGROUND_COLOR },
+          Platform.OS === 'ios'
+            ? { backgroundColor: DEFAULT_BACKGROUND_COLOR }
+            : null,
         ]}
       >
         <SafeAreaView forceInset={forceInset} style={containerStyles}>