Add a test rule for adding notifications

This CL introduces a test rule to add notifications before a test class
and remove them afterward. This can be useful for memory tests focused
on notification memory or jank tests.

Also add additional methods to INotificationHelper interface to post and
cancel notifications.

Bug: 119048359
Test: atest NotificationPressureRuleTest, build internal master
Change-Id: Ie5ebdafde71c387fc82130e9b9905c344b5f36ae