Add closeAllApps scenario and refactor

- Add new test to close multiple apps in desktop mode.
- Refactor flicker scenarios into their own class to avoid duplication.
- Replace DesktopModeUtils with DesktopModeAppHelper wrapper.

Test: atest com.android.wm.shell.flicker.service.desktopmode.flicker
Bug: 316318390
Change-Id: I60f37fb507850daddc3cb91529c272e9f0dbbb44