Android Canned Push Sender

I need a very lightweight Android app whose single purpose is to send one predefined, text-only push notification to all subscribed users whenever I tap a “Send” button. No scheduling, no automated triggers—just a manual push on demand.

High-level flow
• On first run the app should register the device for push via Firebase Cloud Messaging (or another standard push service you prefer).
• In the main (and only) screen I want a simple, clean interface that shows the canned text, plus a prominent button that dispatches it to every subscribed device.
• Subscribers simply install the same APK; as soon as they open it once they’re eligible to receive the notification.

- there also needs to be a simple administrator portal that can allow the subscribed clients to be turned on or off.

Acceptance
If pressing the button reliably delivers the exact text notification to at least two test devices running Android 9+ within a few seconds, the job is done.

Keep it minimal—no images, links, or extra UI.