Messaging: Implement per conversation channels. Update API level to 28.

* Get rid of in-app settings for notifications, replaced by channels features
* Remove all the notification plumbing and dead code

Test: m, manual

Signed-off-by: Luca Stefani <luca.stefani.ge1@gmail.com>
Signed-off-by: Joey <joey@lineageos.org>
Signed-off-by: Danny Baumann <dannybaumann@web.de>
Signed-off-by: Arne Coucheron <arco68@gmail.com>
Signed-off-by: Lyubo <lyubodzhamov@gmail.com>
Change-Id: Idb39ca32751d40b3376934775d2119dd6cc7e297