object: hide strand getter

Change-Id: Ie044ffe4fc77f3e4e1324fd8a8488f1be8d6d502
Reviewed-on: http://gerrit.aldebaran.lan/49799
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Tested-by: pdaouadi <pdaouadi@aldebaran-robotics.com>