Checks keyguard earlier when blocking notifications.

This change modifies NotificationInfo to call the provided keyguard
callback when the "Stop notifications" button is tapped, rather than
when trying to save the new importance value after the undo timeout.

Test: updated test cases in NotificationInfoTest.
Bug: 111416533
Change-Id: I89b4715d0d41be4587753f554862e51c4e1d43c3