Returns whether the user is currently opted-in to receive push
notifications. FCM is the primary channel going forward, so this
reflects FCM's state whenever it's configured for the app; falls back
to OneSignal for apps that don't have firebaseOptions set yet.
Enables push notifications for the end user, across both OneSignal and
FCM - both SDKs stay installed and active permanently (OneSignal is
intentionally never removed, only unused for sending after the FCM
cutover), so a toggle that only touched one would leave the user still
(or no longer) subscribed on the other.