Analytics: When using ScreenTrackingService the average engagement time is calculated the wrong way

### Version info

**Angular:**
"@angular/animations": "13.1.3",
"@angular/common": "13.1.3",
"@angular/compiler": "13.1.3",
"@angular/core": "13.1.3",
"@angular/forms": "13.1.3",
"@angular/platform-browser": "13.1.3",
"@angular/platform-browser-dynamic": "13.1.3",
"@angular/router": "13.1.3",
"rxjs": "7.4.0",
"tslib": "2.3.1",
"zone.js": "0.11.4",
"typescript": "4.5.5"

**Firebase:**
"firebase": "9.6.7",

**AngularFire:**
"@angular/fire": "7.2.1",

**System:**
NodeJS: v12.20.2
npm: 6.14.11
OS: macOS Monterey (12.2.1)

### How to reproduce these conditions

[Demo example](https://github.com/timkos/test-analytics) (fix firebase config in AppModule)

**Steps to set up and reproduce**
1. provide FirebaseApp and Analytics and add ScreenTrackingService to AppModule:
```
imports: [
BrowserModule,
AppRoutingModule,
provideFirebaseApp(() =>
initializeApp(config)
),
provideAnalytics(() => getAnalytics())
],
providers: [ScreenTrackingService] ```
2. Create two routes `/test` and `/test2`. The route `/test` we will visit and stay on it for a longer time. On the other hand, we will always go to `/test2` just for a short time and then always go back to `/test` route.

**Sample data and security rules**

### Debug output
After some time we can see Average Engagement Times for visited paths (screens) on

Firebase -> Analytics -> Dashboard -> View pages and screens.

To see data for screen names (routes) switch from 'Page title and screen class' to 'Page title and screen name'.


### Expected behavior
Average Engagement Time of the route `/test` should be much bigger then the one of the `/test2` route.

### Actual behavior
Average Engagement Time of the route `/test` is much smaller then the one of `/test2` route.

It looks like previous engagement time is added to the next visited route (screen). It should be added to the previous route (screen).

从 AppModule 中的 ScreenTrackingService 注册开始，按照链接演示中的描述访问 /test 和 /test2，重现路由序列。将 Firebase Analytics 的屏幕互动结果与预期的路由归因进行比较；当存续时间更长的 /test 路由报告的平均互动时间显著高于 /test2 时，即表示完成。

由索引模型根据 Issue 内容生成。