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).

Bắt đầu từ việc đăng ký ScreenTrackingService trong AppModule và tái hiện chuỗi route từ bản demo được liên kết, truy cập /test và /test2 như mô tả. So sánh kết quả tương tác với màn hình của Firebase Analytics với việc gán route dự kiến; được xem là hoàn thành khi route /test tồn tại lâu hơn báo cáo thời gian tương tác trung bình lớn hơn đáng kể so với /test2.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.