Fix incorrect easing functions for CardStack (#3381)

* Fix invalid easing functions for CardStack

1. `timing` function accepts `TimingAnimationConfig` which expect `easing` (or dont expect at all) function with following signature: `(value: number) => number`
2. under the hood, `TimingAnimation` falls back to `easeInOut` function, if `easing` is not specified
`this._easing = config.easing !== undefined ? config.easing : easeInOut();`
3. by mistake passing `Easing.linear()` results in following
`typeof Easing.linear(); // undefined` which makes statement in step 2 fall back to `easeInOut`

This commit makes passing `easeInOut` function explicit and fixes existing issue and several Flow warnings

* Do not memoize easing function, keep things simple

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

@@ -22,6 +22,8 @@ import TransitionConfigs from './TransitionConfigs';
 
 const emptyFunction = () => {};
 
+const EaseInOut = Easing.inOut(Easing.ease);
+
 /**
  * The max duration of the card animation in milliseconds after released gesture.
  * The actual duration should be always less then that because the rest distance
@@ -160,7 +162,7 @@ class CardStack extends React.Component {
     Animated.timing(this.props.position, {
       toValue: resetToIndex,
       duration,
-      easing: Easing.linear(),
+      easing: EaseInOut,
       useNativeDriver: this.props.position.__isNative,
     }).start();
   }
@@ -176,7 +178,7 @@ class CardStack extends React.Component {
     Animated.timing(position, {
       toValue,
       duration,
-      easing: Easing.linear(),
+      easing: EaseInOut,
       useNativeDriver: position.__isNative,
     }).start(() => {
       this._immediateIndex = null;