Add ability to show wallpaper during animation

In the new task switch animations, we'd like to show the wallpaper
behind. For that, we add the ability to show the wallpaper for
any animation via Animation.showWallpaper as an XML attribute.

If the window of an app is animating, and the animation requests
the wallpaper, it can also be a wallpaper.

One remaning issue here is that we don't wait for the wallpaper
to be drawn when waiting for the transition. However, usually this
isn't an issue because the wallpaper is drawn in any case.

To fix this we'd need to load the animation already and then make
it a target before the animation starts, which is a bit involved
/quirky.

Test: Open/close task, observe wallpaper behind
Test: go/wm-smoke
Bug: 72396815
Change-Id: I676273a4e6627f8b5e0a1366fcd80a7c92018123