devtools can only access the first created engine when you have multiple engines in add to app

# Overview

When you have a add-to-app with multiple flutter engines active at the same time, the devtools takes the first valid isolate and sets it as `main`, making it impossible to use widget inspector and other tools

Minimal Example App - https://github.com/AshishSingh2001/minimal-add-to-app

## Steps to reproduce

- run the android app
```bash
cd android_app
./gradlew installDebug
```
- Attach flutter debugger to the dart VM service (you can get this from logcat)

## Issue

In the debugger you will always see homeTab and don't have a way to switch to another tab

The widget inspector is build with keeping normal flutter apps in mind, where only one isolate can be the main isolate, but in add-to-app there can be multiple isolates which can render flutter separately

首先，使用 Android 安装步骤和 Dart VM service 连接，通过链接的最小 add-to-app 示例重现该问题。检查多个 Flutter engines 处于活动状态时 DevTools 调试器和 widget inspector 的行为；完成标准是可以选择相关的 engine 或 isolate，而不是始终使用第一个。

由索引模型根据 Issue 内容生成。