- Rename calculateAmount to fetchRate to be more descriptive

- Add Disable Exchange change reaction to save in shared prefs [skip ci]

lib/store/settings_store.dart

@@ -115,6 +115,11 @@ abstract class SettingsStoreBase with Store {
         (BalanceDisplayMode mode) => sharedPreferences.setInt(
             PreferencesKey.currentBalanceDisplayModeKey, mode.serialize()));
 
+    reaction(
+            (_) => disableExchange,
+            (bool disableExchange) => sharedPreferences.setBool(
+            PreferencesKey.disableExchangeKey, disableExchange));
+
     this
         .nodes
         .observe((change) {