Remove padding customization from stack scroller algorithm

Follow up to ag/13004818 - the normal divider height is now 0.5dp =>
4dp, which is not that different when compared to the divider
with increased height at 6dp.

This change simplifies padding math by removing the
- increased padding getter that ExpandableViews override
- padding map from stack scroll algorithm

Bug: 172289783
Test: visual, no regressions
Change-Id: I1183bdbcef81f0bb60bc07d71f9b5bb5f4dce50c