Set up show/hide in startT/finishT for wallpapers

Historically wallpapers weren't specially animated showing/hiding in
shell transitions because the system server would unilaterally update
wallpaper visibility according to the instantaneous request from visible
windows.

As we can have more than one wallpaper window now, and also as we play
some show/hide animations from shell instead of core, setting up the
visibility in sync with startT and finishT prevents flickers in the
handover between shell and core due to reparenting out of leashes at
the end of the animation.

Flag: com.android.window.flags.ensure_wallpaper_in_transitions
Test: atest FlickerTestsAppLaunch2
Test: atest ShellTransitionTests
Bug: 347593088
Bug: 298186160
Change-Id: Ic0524c97f2922437d7ab20fcaa279aa5793ab7ec