Add test for quick switching out of desk.

- Assert that the desktop app is at top at start.
- Assert that fullscreen app is on top at end.

Fix: 418232738
Test: atest QuickSwitchBetweenDesktopAndFullscreenTest
Flag: EXEMPT adding tests
Change-Id: I7d60d525047c0ce6c1553eeb90b559962c31835f