[15/N] Desks: Deactivate desk when last window is minimized

When the last task is minimized, the desk is now deactivated to prevent
future app launches from launching in freeform.

A small refactor is made to move the deactivation and transition
tracking to a utility method.

Note: minimization is not yet migrated to multi-desks, so while the desk
is now deactivated, the minimized task remains visible because it is
only being sent to the back of its desk container, which effectively
keeps it in place since there's no home/wallpaper inside the desk.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 391485148
Bug: 394268248
Test: open fullscreen app, enter desktop, minimize it
 1) DesktopRepository dump shows activeDeskId=null
 2) Home launches
Change-Id: I25eb436595732cf6831f2a64f067cd771e337153