[SurfaceColor] Read bottom sheet color from controller

Remove #getBackgroundColor from BottomSheetContent, and move it to
BottomSheetController. The CL should be no-op regarding product
behavior.

Context: When bottom sheet has a scrim behind in dark mode, using
colorSurface does not create enough contrast with the scrim behind. The
initiative is to make bottom sheet using a lighter color where scrim is
visible behind. This means sometimes bottom sheets can have different
colors.

This CL is in prep to change bottom sheet's background color based on
the content type they are, and their scroll offset.

Bug: 414883705
Change-Id: I291f6333ae54d38272a91024e3230e4c250dbf49
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6578387
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Reviewed-by: Charles Hager <clhager@google.com>
Commit-Queue: Wenyu Fu <wenyufu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466195}