Don't wait until boot timeout if there is no wallpaper.

When launching only core apps, the wallpaper service
is not started.  Without this change the WM waits
up to 30 seconds for the wallpaper window to be created even
though it will never happen.  This introduces a significant
delay before the boot animation is dismissed so the user can
enter a decryption password.

Bug: 6263070
Change-Id: Ia975127a0bf09cf99818f7cc4fd6c0264b740ec6