Use the splash screen background color as the fallback background color

For the widget transition animation fallback background color, use either
the splash screen background color if a task is starting or the task
description-attached background color.

With a background in place, allow the window to fully fade out before
fading in the widget, by moving the window alpha math into an animation
factory-specific implementation.

Bug: 187706750
Test: Manual
Change-Id: I2b5a7783b0585d447ad60534bc48d2e2176877ed