Allow notification actions to be configured.

Android auto requires a larger height for the notification actions.
Allow standard and media notification action containers to be adjusted
accordingly.

In addition, the media buttons need to be laid out with equal spacing
rather than left aligned. Allow the gravity of the media container to be
customized.

Test: booted on phone and Android Auto headunit
Bug: 33210494
Change-Id: I80bb87a478bb8b06184e73d8dc45db0cbbeb932b