DeskClock: Rework notification channel implementation

* Alarm notifications etc. are important and should be on high
  importance level.
* On contrast, upcoming alarms etc. should not have sounds or vibrations
  attached to them.
* In addition, we need to fix strings for notification channel.
* In order to achieve this, we need to create some new channels because
  you can't change the priority for existing channels
* Delete old channels on boot and update the names of existing channels if
  they already exist (they get created with the first notification
  requiring them)
* Move creation of upcoming alarm notifications into one place

Co-authored-by: Wang Han <416810799@qq.com>
Change-Id: I6d2e9abd6a822a62b3313c62b0617d8d9211948e