Use opaque window as reference to show snapshot starting window

A fully transparent window won't display any content on screen.
It also cannot affect insets appearance (system ui flags) so it
should not affect creating starting window. Otherwise the status
bar color may be wrong when the starting window is showing.

Bug: 140811348
Test: atest AppWindowTokenTests#testGetTopFullscreenOpaqueWindow
Test: 1. Click quick search box on launcher.
      2. Dismiss the search view.
      3. Turn off screen and unlock.
      4. Check the text color of status bar.

Change-Id: I54f837295a32d3ab6bb54d2d69aa50d8f50b0fa9