[25/N] Desks: Replace visibleTaskCount checks with active desk checks

Many components use visibleTaskCount as a proxy for whether the system
is currently in desktop mode. This change abstracts that behind a
DesktopRepository#isAnyDeskActive function, which relies on activation
state rather than task count when multiple-desks are enabled.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 362720497
Test: move a task to a desk, verify the taskbar becomes pinned
Change-Id: I8d0798d97444ffcffc8f7d467c1f26a66e989244