Applying possible solution for undesired behavior.

When a stock symbol is added or removed, the change is applied immediately to the current view, however, when the app is restarted, none of the previous changes in stock list SharedPreference exist.

Reported as issue udacity#32 on StockHawk repository.
"Symbol changes are not saving correctly to disk SharedPreferences"
Read for details: udacity#32

app/src/main/java/com/udacity/stockhawk/data/PrefUtils.java

@@ -39,7 +39,7 @@ public static Set<String> getStocks(Context context) {
 
     private static void editStockPref(Context context, String symbol, Boolean add) {
         String key = context.getString(R.string.pref_stocks_key);
-        Set<String> stocks = getStocks(context);
+        Set<String> stocks = new HashSet<>(getStocks(context));
 
         if (add) {
             stocks.add(symbol);