Desks: Check desk exists before attempting to remove it

Sanitize desk-remove requests from Launcher to avoid a SysUI crash if an
unknown desk id is supplied.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 415401023
Test: atest WMShellUnitTests
Change-Id: Iaf5e87fe9bc460c8a71a02fbdff35af57712b763