Desks: Add desk creation limit

Adds a config_maxDesktopWindowingDesks to put a limit on the number of
desks a user can create. The limit is an overall system limit, not per
display. A limit of 0 means unlimited.

This change checks this limit before creating new desks, except when the
request comes as a result of connected display changes, to allow a newly
connected displays to obtain at least 1 desk even if the user was
already at the limit prior to the connection.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 393962589
Test: override the config with the sysprop, try to create more than the
limit and verify it does not succeed
Test: atest WMShellUnitTests

Change-Id: Id52bd3303d8ac0fcf7e06785fafd3d0c218135ea