[Media] Card background.

The part of the card that's behind the foreground and displays the
art work. The composable can display a placeholder while loading, load
the artwork from a callback, and display an overlay above everything.

Bug: 397989775
Test: tested in testbed app with fake data, see b/397989775#comment2 for a screen recording of the entire thing in action in the compose gallery app
Flag: EXEMPT none yet, code is currently unused
Change-Id: I1d07cc4436e59a7ee60d35b5e9dbc0686fbbdbec