Get starting window background color directly.

SystemUI does not need to re-calculate background color of starting
window because Shell library is bound to SystemUI, so status bar can
get background color directly, also this can support to get the
background color from snapshot starting window.

Also fix the starting window listener did not receiveing background
color successful because makeSplashScreenContentView is execute on
another background thread, so there should block the splash screen
thread until the background color is estimated.

Bug: 192213918
Bug: 192659309
Test: manual launch apps from Notification and Launcher, ensure the
color is correctly send to SystemUI or Launcher.

Change-Id: I626cb23d7aebfe4e032fad285bfad374c0e11cea