TopologyScale: limit vertical padding

In the original design, we don't allow the vertical padding to be more
than the tallest display block on the top or bottom. This patch simply
applies that strategy that was in the design.

Test: TopologyScaleTest
Bug: b/352648432
Flag: com.android.settings.flags.display_topology_pane_in_display_list
Change-Id: Icd3e0c93e4201d8251de0be4ca636352115c657d