Desks: Add getOrCreateDefaultDeskSuspending

To allow obtaining a default desk even if one has not being pre-created.
Also migrates |moveTaskToDefaultDeskAndActivate| to use this API as a
first step to migrating all existing usages of the non-suspending API.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 406890311
Bug: 399394443
Test: open fullscreen app, enter desktop from handle menu button -
verify it enters desktop successfully.

Change-Id: Iaa7ac3f18d700096db2c454a2f45434e1d0de884