Unreliable Auth state shared with app extension using same access group

### Environment
* Xcode version: 12.2
* Firebase SDK version: 7.1.0
* Installation method: `CocoaPods`
* Firebase Component: Auth

### Description
After logging out and in again in the main app, `AuthStateDidChangeListener` reports no logged in user in the app extension until the calling app is restarted.

#### Steps to reproduce:
Assume we have a main app and a share extension. The share extension can be called from other apps (e.g. Amazon, Safari).
Implement cross-app authentication with shared iOS Keychain as described here: https://firebase.google.com/docs/auth/ios/single-sign-on
The share extension calls `Auth.auth().useUserAccessGroup()` like the main app does too and listens for the auth state with a `AuthStateDidChangeListener`.
* Log in in the main app
* Check user is also logged in in the share extension (e.g. by sharing a website from Safari)
* Log out in the main app
* Check user is also logged out in the share extension
* Log in in the main app again
* Check user is also logged in again in the share extension

##### After these steps:
Expected: User is also logged in again in the share extension
Observed: User is not logged in in the share extension (user is `nil` in share extensions `AuthStateDidChangeListener`)

##### Weird solution:
Restarting the sharing/calling app (e.g. Safari) resolves the issue and the user is logged in again in the share extension despite the main app wasn't active in the meantime.