[Media] Simplify how we get the background color for the media player.

The color was being passed down all the way from MediaDataManager to
ColorSchemeTransition when it doesn't need to be. This moves the
background color fetch to ColorSchemeTransition, which groups it with
all the other color fetching.

Bug: 228960370
Test: atest SystemUITests
Test: manual: verify media player still looks good
Change-Id: I0a542fd68847b942c3cb1e3c4576344068a06a07