Switching to relative rank comparison to reduce flake risk

Switching testAdjustNotification_rankingScoreKey_notAllowed from
checking absolute ranks to checking relative ranks of two notifications
to reduce risk of ranks changing due to incoming notifications during
tests. The relative ranks should remain the same for the duration of the
test.

Bug: 156557782
Test: atest NotificationAssistantServiceTest
Change-Id: I3f29f283126c2c62d5124989e33a8a605430b91c
1 file changed