Prevent the work mode switch thumb from being dragged.

When we normally press the work mode switch, it waits for the profile to
receive the ACTION_MANAGED_PROFILE_AVAILABLE broadcast before the button
UI actually toggles. The user never interacts directly with that switch.
However, dragging the switch thumb used to bypass that logic. This CL
makes sure dragging the switch thumb cannot happen in the first place.

Change-Id: Ib3b4ae7591136c35e6d02783387d752e246cfb5b
Fixes: 111211805
Test: Manual