Fix incorrect call duration after system time changes

Currently, using system time to calculate elapsed time for call duration.
If we change system time, it will lead to calculate duration incorrectly.
To fix it, using elapsed real time to calculate the duration instead of
the current system time.

Change-Id: Ie3725b87bf9aad747f81c1703343b60f5af8dcb8
CRs-Fixed: 767005