Add show wallpaper feature for back navigation

For the back gesture animation, we want to show the wallpaper but we
don't have any wallpaper targer, so we introduce a new flag to set the
focused window as wallpaper target if it have to show the wallpaper.

Test: Manual with the upcoming CL implementing the cross-task animation
Bug: 241808055
Change-Id: I97b27dc009fe0d017083d16f7117f04d93fb2beb