ParameterMgr: enforce Tuning Mode on write access of parameters

This applies only to setParameter: this commands should only be available in
Tuning Mode. This is enforced by moving the tuning mode check from the remote
interface to the internal CParameterMgr::accessParameterValue() method.
Additionaly, CParameterMgr::accessValue is made private.

Change-Id: I189362a768ce1cd9709b6e5d48cd833f7ca56479
Signed-off-by: David Wagner <david.wagner@intel.com>