Desktop screenshots: Temporary fix to ensure UI is excluded from image

Add a simple delay before taking a partial screenshot so that there is
time for the pre-capture UI to hide before the region image is captured.
This is a temporary solution for the upcoming dogfood release that
should work for 99% of cases until a more robust solution is
implemented.

Bug: 435225255
Test: Manual
Flag: com.android.systemui.large_screen_screencapture

Change-Id: I29ca34dbc9a237537134eeae2db56b32c4d4f91d