Notification improvements for android 8+

[schwma]

• Fix persistent notification causing vibration and audio alerts (for example when loosing connection to server)
• Allow message notifications to pop up as heads-up notifications
• Add LED light to message notifications
• Prevent duplicate notification sounds from message group notification
• Add message notification accent color (set to colorPrimary)

[jmattheis]

Thanks for your contribution, the commits are structured really well and are easy to understand!

Most of the stuff should already be configurable in the android notification settings for the application (right?) tho the used defaults weren't that good. So good change.

I've only one nitpick. The foreground notification doesn't have the blue-ish color that the other notifications have.

[jmattheis]

Fixed my nitpick in #20

[schwma]

Thanks for the merge!

Thats true! However, some notification settings can't be changed on stock Android and most default ROMs without installing additional apps. For example I can't adjust the notification LED settings on my device.