Allow the creation of IMPORTANCE_NONE channels

- Users will still be notified of foreground services
by the system even if the app posts their fg service
notification to a blocked channel so this restriction isn't needed
- This allows apps to cleanly port over their pre-channels notification
settings

Fixes: 62028083
Test: runtest systemui-notification

(cherry picked from commit 3f25f4d6fe24f3782d6e156f9969d98ab3c8b4bf)

Change-Id: I1cd8cfaf31f00088e267bd3fb1bd7912f4397c69