Desks: Deactivate desk on other desk activation

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 404545438
Test: have two desks, move from one to the other using adb command,
verify dumpsys shows it as deactivated. Then go home, new tasks open in
fullscreen.

Change-Id: I2f1765f7bfe7f6e420ca260a045f0eeef0da7c36