Fix enterDesktopWithDrag test

A previous change made this test to fail due to test app not being
launched. Use new enterDesktopMode method instead as it defaults to
enterDesktopWithDrag now.

Fix: 379662054
Test: atest EnterDesktopWithDragPortraitTest
Flag: EXEMPT Bugfix
Change-Id: If8025cd5d601be5c972c0fa5d002cf4a9ccc7837