Fix screenshot preview border radii

Currently the corner radii are the same (20dp) for both the preview
and the border around it. However, this looks wrong because the
border is larger. To match the curve it needs to have 24dp radii
corners (=20 + 4dp border).

Old: https://screenshot.googleplex.com/5SLGsc4Erqus9r6
New: https://screenshot.googleplex.com/Cz7w72BzjgqfqbV

Bug: 190630434
Fix: 190630434
Test: visual verification
Change-Id: I737c4091a607ffdf854d5b7418634f1bdcba75a0