Moves test utils from NLSTest to RankingTest

Moves some test utilities for checking NotificationRanking equality, and
copies some utilities for generating NotificationRankings, from
NotificationListenerServiceTest to NotificationRankingUpdateTest.

Also moves some tests focused on Rankings to NotificationRankingUpdateTest.
These will be deduplicated in the next cl.

Test: Modification of tests only; ran atest on both
Bug: 249848655
Change-Id: I3872396da815368750b5784bc28b6c972d90190c