Guard notifyActionSuccess with flag

Also adds tests and I created a new bug to decide if there's actually
a better course of action to take when our notifications flag has been
turned off AFTER some SC notification(s) is/are already posted.

Test: SafetyCenterNotificationTest
Bug: 283780918
Bug: 284271124
Relnote: N/A
Change-Id: I13a551ac6a8bc2e4fb73a0c9098b49c71e09ff77