FirebaseApp.UserUnlockReceiver.unregister crashes

* Firebase Component: Common
* Component version: 20.4.2

#### Steps to reproduce:

Crashlytics notified us about a crash in `FirebaseApp.UserUnlockReceiver.unregister`. The method is only called from `FirebaseApp.UserUnlockReceiver.onReceive` which in turn only responds to `android.intent.action.USER_UNLOCKED`. I don't know how many times the system sends the broadcast but it apparently isn't just once.

#### Relevant Code:
https://github.com/firebase/firebase-android-sdk/blob/b77c2188e5ec7c53c2a955fc4663b301e6c98c83/firebase-common/src/main/java/com/google/firebase/FirebaseApp.java#L668-L680

`FirebaseApp` initialization may be idempotent but `Context.unregisterReceiver` isn't. The method crashes if the receiver isn't registered.

#### Suggested solution
```java
public void unregister() {
try {
applicationContext.unregisterReceiver(this);
} catch (IllegalArgumentException ignore) {
// The receiver isn't registered.
}
}
```

Start in firebase-common/src/main/java/com/google/firebase/FirebaseApp.java at FirebaseApp.UserUnlockReceiver.unregister and its onReceive caller. Reproduce or inspect the repeated android.intent.action.USER_UNLOCKED path, then verify that an already-unregistered receiver no longer crashes while normal unregistration still works.

Written by the indexing model from the issue text.