[Expressive] Set clipToPadding=false to allow more scrollable area

With b/416089006, we have paddingTop and paddingBottom set on the buttonPanel. If the alertDialog doesn't have enough space for the entire buttonPanel, the scrollable area became too small to effective scroll.
Set clipToPadding=false, would allow increase the scrollable area.

Bug: 406415609
Test: manual (See b/406415609#comment14 for screenshots)
Flag: EXEMPT low risk
Change-Id: I2314db1f260056d62eaeffc22720b8e687e2742a