This is needed for compatibility with targetSdkVersion>=31. Without it,
push messages cause a crash on Android 12 due to missing
FLAG_MUTABLE/FLAG_IMMUTABLE on PendingIntent[0], notification trampoline
issue, and possibly other such reasons.

Note that Firebase has removed the FirbaseInstanceId class in favor of
FirbaseInstallations. To restore it for now, add the firebase-iid
dependency, see [2].

[0] https://developer.android.com/about/versions/12/behavior-changes-12#pending-intent-mutability
[1] https://developer.android.com/about/versions/12/behavior-changes-12#notification-trampolines
[2] https://firebase.google.com/support/release-notes/android#messaging_v22-0-0

Since 1ae3428, when
`PushInstanceIDListenerService` was changed away from the deprecated
`FirebaseInstanceIdService` to use
`FirebaseMessagingService::onNewToken`, there is no longer a need to use
a separate service anymore. Further, the `INSTANCE_ID_EVENT` intent
filter is not needed.

The callback gets the new token already, there is no need to fetch it
again.

These have been deprecated in recent firebase-messaging releases.

See:
https://developer.android.com/guide/components/intents-filters#DeclareMutabilityPendingIntent

Tasks.await throws ExecutionException so it needs to be unwrapped for
the existing error handling to work, otherwise an IOException causes the
app to crash.