Speed up SafetyCenterNotificationTest.

These tests wait for all notifications to go away in teardown. This
includes the success notifications tests. But these notifications
have an auto cancel of 10s, so they wait unecessarily for 10s.
Fixed by cancelling these notifications manually instead.

Also fixes a bug where we would still attempt to show an empty
notification if the source doesn't specify an error message
(we should have either a default success message or skip sending
a notification). I realized this because the
`successfulNoSuccessMessage` test would still take 10s to complete.

Bug: 293611291
Test: atest SafetyCenterNotificationTest
Relnote: Fix an issue where an empty notification shows when
resolving issue from Safety Center notification

Change-Id: I24d963ffe0c477a76d419ff7675f60cc814838bf