Fix notification tests

- Fix method incorrectly marked test
- Fix access to non-@hide methods
- Fix a logic error in Notification.java
- Move the 'core' notification test that were in the services suite to
the 'core' suite

Test: atest NotificationTest
Fixes: 254459150
Change-Id: Ib2793badb6b0616097bb837d7fd2c7504a43e186