Why does Firebase pendingAuthResult return null when the app is killed?

### Step 1: Describe your environment

* Android device: Xiaomi Mi A1
* Android OS version: Android 8.1.0
* Google Play Services version: 4.3.3
* Firebase-core version: 17.2.1
* Firebase-auth version: 19.2.0
### Step 2: Describe the problem:

When the app in the background is killed while performing Apple sign-in in Chrome, the authentication behaves inconsistently.

#### Steps to reproduce:
1. Go to `GenericIdpActivity`
2. Start the Apple sign-in flow
3. Before finishing the sign-in flow, terminate the app in Android Studio to simulate system killing the app
4. Finish signing in
5. Both `auth.currentUser` and `auth.pendingAuthResult` are `null` in `onStart()`

#### Observed Results:
If I then restart the app, I am **suddenly** signed in and the pending result is present. And it doesn't disappear no matter what I do.

#### Expected Results:
The pending result shouldn't be `null` if the app is killed while in the background. That is the whole point of the pending result. **Generally, the current behavior when the app is killed seems really inconsistent and buggy.**

#### Related SO question:
https://stackoverflow.com/questions/59718353/why-does-firebase-pendingauthresult-return-null