Ignore implicit high priority for 'hide silent notifs on lockscreen'

Normally, HighPriorityProvider is willing to "promote" low priority
notifications to high priority if they "have high priority
characteristics" (are media/conversation/messaging and aren't *user* set
as low priority).

This is fine for ranking, but not fine for deciding which notifications
to show on the lockscreen, so add a distinction between (potentially
implicitly) high priority and *explicitly* high priority, and use
*explicit* high priority when making lockscreen visibility decisions.

Bug: 286278782
Test: atest HighPriorityProviderTest KeyguardNotificationVisibilityProviderTest
Change-Id: I933d78eed841ded452de126919898e1dfb0d0e35