app: On launch, go to the last visited account

Fixes: #524

Author: sm-sayedi

lib/widgets/app.dart

@@ -211,13 +211,12 @@ class _ZulipAppState extends State<ZulipApp> with WidgetsBindingObserver {
     }
 
     final globalStore = GlobalStoreWidget.of(context);
-    // TODO(#524) choose initial account as last one used
-    final initialAccountId = globalStore.accounts.firstOrNull?.id;
+    final lastAccountId = globalStore.lastVisitedAccountId;
     return [
-      if (initialAccountId == null)
+      if (lastAccountId == null)
         MaterialWidgetRoute(page: const ChooseAccountPage())
       else
-        HomePage.buildRoute(accountId: initialAccountId),
+        HomePage.buildRoute(accountId: lastAccountId),
     ];
   }