Add Tron logging of explanation for notification importance.

Adds the results of CalculateImportance by default on notification
logs, so that we can track the effects of NotificationAssistant and
user and system importance overrides.

Adds tests for the various different sources of importance.

Bug: 127834982
Test: atest frameworks/base/services/tests/uiservicestests/src/com/android/server/notification SystemUITests

Change-Id: I044d1206f5180a028bb55704ced05117bc0a00bb