Desks: Check desks exists before attempting to activate it

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

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 416166930
Test: atest WMShellUnitTests
Change-Id: I20da7ab70c7a57932acb7e3e9930e7a46bbf7318