Fix wallpaper flicker issue while transiting from home to aod.

There is a race condition between window visibility and wallpaper
transition, so wallpaper window might become visible before background
thread has done its rendering work and a flicker happens.

This solution just lets main thread wait for background thread to
synchronize window visibility and wallpaper transition.

Bug: 136643341
Test: atest SystemUITests
Test: Screen off  Launcher/APP -> AOD : No extra transition by design,
      The duration between press Power Key and AOD show should be same.
Test: Screen on AOD -> Launcher : Face unlock work without flickr
Test: Screen off Keyguard -> AOD : Either Panel or background transition without flickr
Test: Screen on AOD -> Keyguard : Either Panel or background transition without flickr
Test: Reach in AOD -> Keyguard : Either Panel or background transition without flickr
Test: Reach out Keyguard -> AOD : Either Panel or background transition without flickr
Test: Manually operate and observe log.
Change-Id: Iebfdac18972569864c047bc2c4a33d7791940896