Refactor DesktopTasksController to update logic for moving task to desktop mode.

- Move allFocusedTasks to a private function
- Move splitFocusedTask to a private function
- Move moveToDesktopFromNonRunningTask to moveBackgroundTaskToDesktop
- Move addMoveToDesktopChangesNonRunningTask to updateDesktopTasksToFront
- Move moveToDesktop to moveRunningTaskToDesktop

Change-Id: I20f50bdcfbed68ab522ec6f10d5554a36716ef20
Bug: 332682201
Flag: EXEMPT (no-op for functionality)
Test: atest DesktopTasksControllerTest