Add a flicker test for desktop apps switching using Alt + Tab

The flicker test, AltTabSwitchInDesktopModeFlickerTest, opens 3 apps in
desktop mode and switches to the first opened app, using the existing
scenario, AltTabSwitchInDesktopMode. It then verifies that the
first app window is on top at the end of the test, and all 3 app layers
stay visible throughout the test execution.

The test is added to a new subfolder, keyboardshortcuts.

Test: atest AltTabSwitchInDesktopModeFlickerTest
Bug: 425918059
Flag: EXEMPT, not needed for e2e tests
Change-Id: Ied71d60f2273dd5a04cb05f7da9399e6002a17a4