call: Add getCallDuration() method

If `duration_start_` is `time_point::min`, then when calculating the call's
duration, there's an signed integer overflow which result in an enormous time
duration.

Signed-off-by: Olivier Dion <olivier.dion@savoirfairelinux.com>
Change-Id: I3ad119ab0f79ef8e9c537d1879d628a29ceed4d7