Skip drag-to-exit-desktop test when drag-to-maximize is enabled

This CL skips ExitDesktopWithDragToTopDragZone when the drag-to-maximize
is enabled on the device.
Otherwise, the test fails on such a device as the consequence of
“DragToTopDragZone” is different (exit-desktop vs. being maximized).

Bug: 376593075
Flag: EXEMPT test assumption fix
Test: ExitDesktopWithDragToTopDragZoneTest
Change-Id: I57cc914d37a7da8e33b29ca11d64d39d6fbba623