Set FCM priority to HIGH

[p1gp1g]

With normal priority, applications are not automatically wake from doze mode
Cf. https://firebase.google.com/docs/cloud-messaging/concept-options#setting-the-priority-of-a-message

We should set the priority with the legacy API and HTTPv1 API to avoid issues (https://stackoverflow.com/a/59939658)

{
    "priority": "high", // legacy HTTP protocol (this can also be set to 10)
    "android": {
        "priority": "high" // HTTP v1 protocol
    }
}