The quietening round 2 part 1

Allow the notification assistant to block or silence
incoming notifications, or demote notifications after they
are posted

Also temporarily silence everything by default

Plus: bonus refactoring of the cancel notification runnable so I could
write just one of those tests :)

Bug: 111475013
Test: manual, runtest systemui-notification
Change-Id: Ifa04a21919f60d06080cd63e7d7747180b641308