Rename Modifier.background(Color, () -> Float) to fadingBackground (1/2)

This CL renames the overload Modifier.background() meant to be used when
the background fades away to Modifier.fadingBackground(). This avoids
people using this when they can simply use Modifier.background.

Bug: 375119446
Test: N/A simple renaming
Flag: EXEMPT simple renaming
Change-Id: Idd18c646ae0397be5c1e8e4047d7cd96c6438b17