Several improvements to StackNavigation Header (#3568)

* Refactor to remove unused variables, styles, and outer Animated view

* Style cleanup

* Proof of concept blur background

* Clean it up and add flow interface

* Update snapshots

flow/react-navigation.js

@@ -381,6 +381,7 @@ declare module 'react-navigation' {
 
   declare export type NavigationStackScreenOptions = NavigationScreenOptions & {
     header?: ?(React$Node | (HeaderProps => React$Node)),
+    headerTransparent?: boolean,
     headerTitle?: string | React$Node | React$ElementType,
     headerTitleStyle?: AnimatedTextStyleProp,
     headerTitleAllowFontScaling?: boolean,
@@ -393,6 +394,7 @@ declare module 'react-navigation' {
     headerPressColorAndroid?: string,
     headerRight?: React$Node,
     headerStyle?: ViewStyleProp,
+    headerBackground?: React$Node | React$ElementType,
     gesturesEnabled?: boolean,
     gestureResponseDistance?: { vertical?: number, horizontal?: number },
     gestureDirection?: 'default' | 'inverted',