Add test for stack scroller padding.

Adding tests before changing the KeyguardClockPositionAlgorithm to deal
with positioning the stack scroller when a full screen custom clock is
present. The tests should help reduce the risk of regressions.

Bug: 120037316
Test: KeyguardClockPositionAlgorithmTest passes locally
Change-Id: I505d9402fd73697f4222138bd753bdfe32f87337