Updated NotificationHelper to use highest notification priority (PRIORITY_MAX) (#203)

We should ensure that notifications always have the highest priority also on older devices.

Co-authored-by: Johannes Münichsdorfer <johannes.muenichsdorfer@tum.de>
Co-authored-by: Jakob Möller <jakob.moeller@sap.com>
Co-authored-by: Hee Tatt Ooi <hee.tatt.ooi@sap.com>