A service tracking the current focus, and guaranteeing that only one client
has the focus at the same time.
The focus is required for actions to be performed on the robot. This
mechanism ensures the focus owner that it can be the only one to control
the robot as long as its FocusOwner is not released.
Take the focus from the given token.
If the token matched the one of the current focus, the ownership is
transmitted to the taker. Any user having the FocusOwner would be released.
Otherwise, it behaves like takeFocus.
token - The token authenticating the focus ownership.