Re-enable and fix failing tests in NotificationStackScrollLayoutTest

Fixing some minor issues with tests.
Deleting the testAppearFractionCalculation(), because its content is no
longer relevant: we just return a value from
AmbientState#getAppearFractionCalculation().

Fixes: b/284136971
Test: atest NotificationStackScrollLayoutTest
Change-Id: Ie215aba0eb764c60e05fcf1baf96cc38b262dd25