Small fixes to CarSysUI Volume Dialog

Make background transparent and makes the volume items black background.
This is to deal with some UI jank where the background appears before
the item appears.

Separated the left and right padding so that they can be overlayed
separately.

Bug: 129402464
Test: manual
Change-Id: Icff89e0858e9b6b73696082ac2fe37fd5cbcaf1c