Fix the flickering of drag-to-split visual indicator

Instead of early return for an event outside of the display area,
extend the drag-to-split regions outside of the display area.
This is needed to not show the indicators for events outside of the
display, but around the top corners of the display.

Bug: 425796298
Test: atest DesktopModeVisualIndicatorTest
Flag: EXEMPT small bug fix
Change-Id: I69711ea0398b5f33016c714ec2a961162bce4cf4