[7/N] Desks: Implement desk activation

Adds a new DesktopTasksController#activateDesk to focus the given desk
and its tasks.

The existing #showDesktopApps used by Launcher maps to #activateDesk
with the default desk in the given display, and the #activateDesk shell
adb command is also wired to activate the given desk.

A small refactor of #bringDesktopAppsToFront is needed now that manually
bringing every one of the desk's tasks to the front isn't needed, and
instead the desk root container is the one being sent to front.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 390692038
Test: adb shell dumpsys activity service SystemUIService WMShell
 \ desktopmode activateDesk <deskId>
Test: selecting desk overview tile activates the desk

Change-Id: I92a9a4675e9d605fa6ffd4d5df375f8cc6e0740e