[Expressive] update minHeight of dialog's button bar

For dialogs with a long content area, the button bar may have a scroll
bar. This happens when the height of the buttons exceeds the minHeight
property of the button bar, causing the buttons to overflow and the
scroll bar to appear.

Bug: 433457325
Bug: 406415609
Test: visual verify
Flag: EXEMPT bugfix
Change-Id: If52405b7525818e97764740b46f31ab2d1ba445d