Move the legacy MyFirebaseMessagingService.onNewToken() device-registration
path onto the new iOS-parity FcmService. The legacy service is already
unwired from the manifest MESSAGING_EVENT filter; this removes its last
functional responsibility. Behaviour preserved: auth-token guard,
DeviceRegistrationRequest with ANDROID_ID + Build.MODEL, log-only on error.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>