Better icon for notifications, avoid foreground service

Summary:
Remove foreground service permission

Stop using singleton for MQTTClient, since each is bound to a service

Reviewers: valerio.bozzolan

Reviewed By: valerio.bozzolan

Subscribers: valerio.bozzolan, Libre BusTO

Tags: Libre BusTO

Differential Revision: https://gitpull.it/D156