Fix conference call duration issue

The conf call start time is updated with a delay resulting
in abnormal display of the conf call duration.
Fixed by updating the duration only when the call start
time is non-zero.

Change-Id: I6a1b6c0ab0021d6e6e260bf1743487ec364366c4
Signed-off-by: Narendra <nagarw@codeaurora.org>