Desks: Apply activation changes on user switch from Shell

Handle transition requests for user switches to deactivate desks of the
outgoing user and activate desks of the new user if applicable.

This also makes changes in DesksTransitionObserver: user-switches are
no longer an "independent" desk transition, so this flags off a lot of
the user-switch detection that was needed before.

Flag: com.android.window.flags.apply_desk_activation_on_user_switch
Bug: 420858253
Bug: 430988310
Fix: 415381304
Test: switch users:
- from empty desk to empty desk
- empty desk to non-empty desk (and viceversa)
- empty desk to Home (and viceversa)
- non-empty desk to Home (and viceversa)
verify tasks are restored as expected

Change-Id: I99a8a243e69d1a4c53606f72fcdc5e511722cbc1