Support for NotificationAssistantService to rerank notifications.

NotificationAssistantService passes a RankingScore via an Adjustment to
NotificationRecord. NotificationManagerService sorts notification using
rank as part of NotificationComparator.

Bug: 146443378
Test: atest NotificationComparatorTest
Change-Id: I9f4d03c7d0ccf5adba4f26f48eb2638aa6caa49b