Don't take screenshot when no WSA surfaces are visible.

Currently screenshots are taken even when no WSA surfaces are visible.
This results in black screenshots geting taken and later used to show in
recents. Restore previous behavior where screenshots are not taken for
apps that don't have any WSA surfaces visible.

Test: Enter split screen. Put an app in the secondary screen. Exit
split screen and enter recents. App that was secondary should now contain
a correct screenshot instead of displaying a black screenshot.

Change-Id: I1417afa0bf6602f3ca7ed55e96fc94bce5f5b4fe