Update ActionButtonsPreference style

 - Add rounded background
 - Add divider between buttons
 Screenshots:
   https://screenshot.googleplex.com/5b3fanqgpbpufyD.png
   https://screenshot.googleplex.com/5jG85UeVk64pAMg.png

Bug: 187923403
Test: atest ActionButtonsPreferenceTest
Change-Id: I04bebbfc676c5c5d019a4dce1b8e31bfc69360d7
7 files changed