Rename fadingBackground to animatedBackground with () -> Color param

This CL renames fadingBackground() to animatedBackground() and makes the
color parameter be a () -> Color, so that the color can also be
animated. This will be used by Expandable so that its color value is
read during the drawing phase.

The implementation is also now based on the Modifier Node API, and is
mostly a fork of the background() modifier implementation.

Bug: 402059261
Test: FooterActionsScreenshotTest
Flag: EXEMPT simple refactoring
Change-Id: Id4751ddb690b4c94d2cb669141cd27a2eb349b56