App layer now stays visible with restart dialog

Before, splash screen replaces app layer when app restarts, so app layer
passes then().isInvisible check. Now, when app is transitioning to exit
size compat mode, app window in size compat mode is exiting with
animatingExit: true and new app window replaces it. App layer stays
visible throughout without splash screen layer transition.

Add check for letterbox layer and don't allow letterbox education to
appear during app compat tests to avoid interfering with app
layer/window visibility checks.

Fix: 275374772
Test: atest WMShellFlickerTests:OpenAppInSizeCompatModeTest
      atest WMShellFlickerTests:RestartAppInSizeCompatModeTest
Change-Id: I173f820593b76a34c2a25eaa116232c955adafe3