Add test for switching out of desktop with Alt+Tab

- Added a functional test to switch out and into desktop mode from
  fullscreen app.

Bug: 382769617
Test: atest WMShellFlickerTestsDesktopMode:com.android.wm.shell.functional.AltTabSwitchOutDesktopModeTest
Flag: EXEMPT adding functional test
Change-Id: Idcdd77538f2aed0ee3e4feb8f2a855571562c44d