Implement `isInDesktopMode*()`

This CL does the following:
- Adds new methods `isInDesktopMode()` and
  `isInDesktopModeAndNotInOverview()` to `DesktopVisibilityController`.
- These two methods rely on the new multi-desks impl when the flags are
  enabled.
- Makes the existing `areDesktopTasksVisible()` and
  `areDesktopTasksVisibleAndNotInOverview()` private, and migrates all
  the usages to the new methods.
- The new methods uses the old methods if the flags are disabled.

A companion NexusLauncher CL is at ag/31601363.

Bug: 394182435
Test: m
Flag: com.android.window.flags.enable_multiple_desktops_frontend
Flag: com.android.window.flags.enable_multiple_desktops_backend
Change-Id: I29ff38f984bfe2f68a120f84e1a75fa36b739258