refactor: split app wrapper from focused example code (complex apps)

Convert the final 12 multi-screen apps to the validated pattern:
lib/main.dart keeps only the MaterialApp bootstrap (plus firebase init where
that is genuine bootstrap), and a new lib/example.dart holds the entry screen
and focused example code, importing existing supporting files. firebase_crash
keeps its runZonedGuarded bootstrap in main.dart and extracts only CrashApp.

Apps: bmi_calculator, covid19_mobile_app, expense_planner,
firebase_crash_reporting, firebase_google_authentication, lunch_app,
navigation_drawer, news_memes_app, save_data_locally_with_sqlite,
todo_list_using_provider, unit_testing, using_firebase_db

unit_testing/lib/main.dart

@@ -2,6 +2,8 @@ import 'package:flutter/material.dart';
 import 'package:unit_testing/screens/home_screen.dart';
 import 'package:unit_testing/screens/sign_in_screen.dart';
 
+import 'example.dart';
+
 void main() => runApp(MyApp());
 
 class MyApp extends StatelessWidget {
@@ -13,6 +15,7 @@ class MyApp extends StatelessWidget {
       debugShowCheckedModeBanner: false,
       title: 'Material App',
       initialRoute: "/",
+      home: const Example(),
       routes: {
         "/": (context) => SignInScreen(),
         "/homeScreen": (context) => HomeScreen(),