Consolidate Desktop Display State Checks

Introduces ShellDesktopState to consolidate logic for determining
display eligibility for desktop mode interactions.

DesktopModeWindowDecorViewModel now correctly consider if the display
has active freeform tasks OR if Home is focused on a desktop-first
display when determining eligibility for window drag, by leveraging the
new checker class.

Bug: b/383069176
Test: manual and atest
Flag: com.android.window.flags.enable_block_non_desktop_display_window_drag_bugfix
Change-Id: I9e994e858cf12cea0704aba2231d9d6352e2b30b