[flow] Fix TabBar types (react-navigation#5800)

1. `TabBarTop` is `MaterialTopTabBar` now
2. `MaterialTopTabBar` does not have a `getButtonComponent` prop (accidentally added in react-navigation#5648)
3. `BottomTabBar`'s `getButtonComponent` prop is a `React.Component`, not a `React.Node`

Author: Ashoat

flow/react-navigation.js

@@ -1184,7 +1184,7 @@ declare module 'react-navigation' {
   };
   declare export var TabView: React$ComponentType<_TabViewProps>;
 
-  declare type _TabBarTopProps = {
+  declare type _MaterialTopTabBarProps = {
     activeTintColor: string,
     inactiveTintColor: string,
     showIcon: boolean,
@@ -1205,12 +1205,20 @@ declare module 'react-navigation' {
       jumpToIndex: (index: number) => void,
     }) => void,
     renderIcon: (scene: TabScene) => React$Element<*>,
-    getButtonComponent: (scene: TabScene) => React$Element<*>,
     labelStyle?: TextStyleProp,
     iconStyle?: ViewStyleProp,
   };
-  declare export var TabBarTop: React$ComponentType<_TabBarTopProps>;
+  declare export var MaterialTopTabBar: React$ComponentType<
+    _MaterialTopTabBarProps
+  >;
 
+  declare type _BottomTabBarButtonComponentProps = {
+    onPress: () => void,
+    onLongPress: () => void,
+    testID: string,
+    accessibilityLabel: string,
+    style: ViewStyleProp,
+  };
   declare type _BottomTabBarProps = {
     activeTintColor: string,
     activeBackgroundColor: string,
@@ -1234,7 +1242,9 @@ declare module 'react-navigation' {
     }) => void,
     getTestIDProps: (scene: TabScene) => (scene: TabScene) => any,
     renderIcon: (scene: TabScene) => React$Node,
-    getButtonComponent: (scene: TabScene) => React$Node,
+    getButtonComponent: (
+      scene: TabScene
+    ) => React$ComponentType<_BottomTabBarButtonComponentProps>,
     style?: ViewStyleProp,
     animateStyle?: ViewStyleProp,
     labelStyle?: TextStyleProp,