android nav: Use a better stack-nav animation.

In one of the recent React Navigation upgrades, the animation for a
new screen entering and leaving the stack, on Android, changed.

We like how it was before [1]. So, put it back.

[1] zulip#4249 (comment)

src/nav/AppNavigator.js

@@ -1,5 +1,6 @@
 /* @flow strict-local */
-import { createStackNavigator } from 'react-navigation-stack';
+import { Platform } from 'react-native';
+import { createStackNavigator, TransitionPresets } from 'react-navigation-stack';
 
 import AccountPickScreen from '../account/AccountPickScreen';
 import RealmScreen from '../start/RealmScreen';
@@ -69,6 +70,12 @@ export default createStackNavigator(
     sharing: { screen: SharingScreen },
   },
   {
+    defaultNavigationOptions: {
+      ...Platform.select({
+        android: TransitionPresets.FadeFromBottomAndroid,
+        ios: TransitionPresets.SlideFromRightIOS,
+      }),
+    },
     initialRouteName: 'main',
     headerMode: 'none',
   },