Fix black frame flicker in UNOCCLUDE transition.

When an activity which occludes the keyguard finishes, WindowManager
immediately updatees the keyguard occluded status, so show-when-locked
wallpaper was not used anymore. However sysui needed some time to finish
keyguard occluded status change in their side, and meantime no wallpaper
target existed.

With this change, we keep using show-when-locked wallpaper until app
transition finishes.

Bug: 201560359
Test: manual. Show assist app on the lockscreen and close.
Change-Id: I4958a2f5ecdd12b3db43b44ee4a221972070ab2c