Wait for visible undrawn wallpaper before turning on screen

In case the wallpaper is redrawing slower than keyguard or the
top activity.

Bug: 326212326
Test: Add sleep 600ms before calling finishDrawing in WallpaperService.
      Change display size while screen is off (e.g. fold/unfold).
      After screen is turned on, the wallpaper should not be black.
Change-Id: I8868b685dadd482c2212d0253b16bfba428ea110