Fix header elevation of NavigationExperimental

Summary:
Updated according to the documentation:  http://www.google.com/design/spec/what-is-material/elevation-shadows.html#elevation-shadows-elevation-android-
Closes https://github.com/facebook/react-native/pull/7289

Differential Revision: D3390296

Pulled By: ericvicenti

fbshipit-source-id: 4c475a6c4b6cd1c40b4d995ed18d31d9e179a69a

Libraries/CustomComponents/NavigationExperimental/NavigationHeader.js

@@ -220,7 +220,7 @@ const styles = StyleSheet.create({
     backgroundColor: Platform.OS === 'ios' ? '#EFEFF2' : '#FFF',
     borderBottomColor: 'rgba(0, 0, 0, .15)',
     borderBottomWidth: Platform.OS === 'ios' ? StyleSheet.hairlineWidth : 0,
-    elevation: 2,
+    elevation: 4,
     flexDirection: 'row',
     height: APPBAR_HEIGHT + STATUSBAR_HEIGHT,
     justifyContent: 'flex-start',