Do not notify desk mode changes on visible desk tasks count changes

1. Trigger `notifyIsInDesktopModeChanged` on
   `visibleDesktopTasksCount` changes only when the multi-desks
   flags are disabled.
2. Make `DesktopVisibilityController.isInDesktopMode` variable
   private and legacy, which should only be used when the multi-desks
   feature is disabled. All the call sites should go to check
   `DesktopVisibilityController.isInDesktopMode(displayId)`, which
    works both with and without the multi-desks feature enabled.

Bug: 402222877
Test: m
Flag: com.android.window.flags.enable_multiple_desktops_frontend
Flag: com.android.window.flags.enable_multiple_desktops_backend
Change-Id: I291e2be6d99e81b0c918ae7f39c17a8cd4ea0918
5 files changed