Clean-up unnecessary desktop windowing flag usage

- mDesktopVisibilityController will now be null if desktop windowing flag is disabled
- As a result, all code that uses mDesktopVisibilityController no longer need to check desktop windowing flag
- DesktopTaskView/DesktopTask only populates when flag is enabled, so we don't need to check flag if we already check DesktopTaskView/DesktopTask
- Removed the flag check in SplitToWorkspaceController, as we shouldn't enable split from workspace base on desktop windowing, contextual split already have its own flag
- Also avoid creating GroupTask or DesktopTask for freeform tasks when flag is off, to avoid a crash in RecentsView

Bug: 332655617
Test: manual with desktop windowing flag on/off
Flag: ACONFIG com.android.window.flags.enable_desktop_windowing_mode DEVELOPMENT
Change-Id: I0e45f73ad2b0d99dfb4ec3215f973679ca93957c