Introduce the FadingBackground modifier

This CL introduces an overload of Modifier.background which takes a
`alpha: () -> Float` parameter to set the alpha of the background. This
should be used when the alpha of the background is animated, to avoid
recompositions during an animation.

The content of FadingBackground.kt was mostly copy pasted from
Background.kt [1].

[1] http://cs/androidx-platform-dev/compose/foundation/foundation/src/commonMain/kotlin/androidx/compose/foundation/Background.kt

Bug: 242040009
Test: Manual
Change-Id: I3296f7bc7382e2afe77a088cfdbefbfae134051d