Initialize transition layer of wallpaper at bottom

Otherwise the wallpaper may be shown above the app during
transition. The case may happen when performing seamless
display change transition with visible wallpaper.

Bug: 229701688
Test: atest com.android.server.wm.flicker.launch.OpenAppColdTest
Change-Id: Ib7b448caec6d1c1f1383b7e4229394437afd1b58