Do not create desks in invalid displays

Prevents creation of desks when the request is for an INVALID_DISPLAY.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 362720497
Test: atest WMShellUnitTests
Change-Id: Id5cda975a5f4d1ca9a20c57e2dd5f26fdbc3b029