Splitted the "get active session" and "activate session"

Splitted the "get active session" and "activate session"

Previously "Activate_NVR_Session" returned the previously
active NVR session. This complicated setup as it was not
possible to simply use "Activate_NVR_Session" command in
the NVR setup procedure but the internal state had to be
set directly. But the feature (returning the previously
active session) was not used for anything.

The new way of doing things is to have a separate keyword
for getting the active session and to have the
"Activate_NVR_Session" setting the internal state but not
attempting to read it so it can be used in the setup call,
improving encapsulation of the internal state handling.

Change-Id: Ie40e086b8cc9054f67a9be4a7fc29b8c962d3db3
Signed-off-by: Jozef Behran <jbehran@cisco.com>