Check importance to determine if notif sounds

If the user has a setting to not show silent notifications on the
lockscreen, then all notifications that don't make noise from all
notification sections aside from the media section should hide. For
example, silent notifications categorized in BUCKET_PEOPLE.

Test: atest NotificationLockscreenUserManagerTest
Fixes: 152934785
Change-Id: I98c0f0d0ac17463f92cc0228903d13ab226c5c1a