Add config flag to specify if wallpaper is rendered

DisplayContent#checkWaitingForWindows is used to block turning on
display at boot until wallpaper is ready. However when Android runs in a
container, wallpaper can be rendred outside of the container. In this
case DisplayContent#checkWaitingForWindows cannot find wallpaper window
and it blocks boot until timeout.

The CL adds config_checkWallpaperAtBoot flag to let
checkWaitingForWindow skip wallpaper check.

Bug: 38394691
Test: Check boot on marlin
Change-Id: I1d051521a5be06a4d773780de6fdc481d09acb95