Provide a sticky notification for toggling demo mode on Marshmallow

[davidschreiber]

Followup for #3

Since the app also works on Marshmallow (API 23) but quick setting tiles were introduced in API 24, the app could provide a sticky notification for toggling demo mode on API 23 devices.

[tasomaniac]

Here is also another hack to add Tiles to Quick Settings even in Marshmallow

http://www.androidpolice.com/2015/10/30/hands-on-custom-quick-settings-adds-new-tiles-to-the-android-6-0-quick-settings-via-ui-tuner/