Do now allow current user to be removed

It was possible to remove a foreground user using pm remove-user command.
The system ends up in the inconsistent state, because switch does not happen
and the removed user stays in the foreground, but its state is removed.

Also added am get-current-user command.

Change-Id: Ida2dce8f99bac55e106cdd44c93e96cc9142d7fb