Add color layer to TaskDisplayArea for animations

We want a solid color background to be used for task animations so that we show that instead of the wallpaper.

We need a way to set the color when the animation start and clear it at
the end of the animation.

Test: atest FlickerTests:TaskTransitionTest
Bug: 199507257
Change-Id: I47fe530ffe37b453bbec8e65e0a4199fba2f04a3