fix: change opacity for hidden tabs only when not using rn-screens (#80)

When using react-native screens we don't need to hide invisible tabs using opacity. This in conjunctions with "active" property normally used by screens where causing blinking effect when new tabs got activated as in some cases opacity would update in a different UI transaction (this would only surface on Android).

This change removes the use of `opacity` style when react-native-screens are active and solely relies on `active` property in that case. When rn-screens are off we fallback to rendering `View` and use `opacity` as before (this is now done in `ResourceSavingScene`).

### Motivation

Fix blinking effect on Android when switching tabs [#5382](https://github.com/react-navigation/react-navigation/issues/5382)

### Test plan

1) Run RN-screens sample app with and w/o screens enabled or try the code from this snack -> https://snack.expo.io/rklSkM-xE
2) Try this on a low end Android device for consistent repro
3) Switch between tabs and see the content disappear for one frame with screens ON when this change isn't active and see this issue gone with this change applied

packages/tabs/src/navigators/createBottomTabNavigator.js

@@ -112,10 +112,7 @@ class TabNavigationView extends React.PureComponent<Props, State> {
             return (
               <ResourceSavingScene
                 key={route.key}
-                style={[
-                  StyleSheet.absoluteFill,
-                  { opacity: isFocused ? 1 : 0 },
-                ]}
+                style={StyleSheet.absoluteFill}
                 isVisible={isFocused}
               >
                 {renderScene({ route })}

packages/tabs/src/views/ResourceSavingScene.js

@@ -24,7 +24,7 @@ export default class ResourceSavingScene extends React.Component<Props> {
 
     return (
       <View
-        style={[styles.container, style]}
+        style={[styles.container, style, { opacity: isVisible ? 1 : 0 }]}
         collapsable={false}
         removeClippedSubviews={
           // On iOS, set removeClippedSubviews to true only when not focused