Move focused task when using taskId 0

This is a quality of life improvement when moving tasks to desks. Using -1 is not permitted because the command interprets it as a flag.

Fixes: 336268819
Test: adb shell dumpsys activity service SystemUIService WMShell desktopmode moveTaskToDesk 0
Flag: EXEMPT small refactor
Change-Id: Id36bcbbc91c7200e14f95b040d8380a42c0aa710