Prune legacy data from Cliqz browser (#371)

cliqz.js

@@ -2,12 +2,27 @@ import 'react-native/Libraries/Core/InitializeCore';
 import './setup';
 import 'process-nextick-args';
 import React from 'react';
-import { AppRegistry, StyleSheet, View } from 'react-native';
+import { AppRegistry, StyleSheet, View, AsyncStorage } from 'react-native';
 import { startup, components } from 'browser-core-cliqz-ios';
 
+async function cleanUpStorage() {
+  const migrateKey = '@migrated';
+  const migrateVersion = '1';
+  const migrated = await AsyncStorage.getItem(migrateKey);
+  if (migrated !== migrateVersion) {
+    console.log('Migrate legacy storage');
+    const keys = await AsyncStorage.getAllKeys();
+    // prune legacy fs and anti-tracking storage namespaces
+    const pruneKeys = keys.filter(k => k.startsWith('@fs:') || k.startsWith('@cliqzstorage'));
+    await AsyncStorage.multiRemove(pruneKeys);
+    await AsyncStorage.setItem(migrateKey, migrateVersion);
+  } 
+}
+
 // set app global for debugging
 const appStart = startup.then((app) => {
   global.app = app;
+  cleanUpStorage();
 });
 
 const styles = StyleSheet.create({