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).

Beginne bei der Registrierung von ScreenTrackingService in AppModule und reproduziere die Routensequenz aus der verknüpften Demo, indem du /test und /test2 wie beschrieben besuchst. Vergleiche die Ergebnisse des Screen-Engagements von Firebase Analytics mit der erwarteten Routenzuordnung; abgeschlossen ist die Aufgabe, wenn die länger aktive Route /test eine deutlich höhere durchschnittliche Interaktionszeit als /test2 meldet.

Vom Indexierungsmodell aus dem Issue-Text verfasst.