Track conference start in system elapsed time.

This is important so that Telecom can track the duration of the call,
which is tracked using wall clock time.

Test: Manual, unit test
Bug: 64068300
Merged-In: If642d282cd8134060acf6ffe8d81215c394d800c
Change-Id: If642d282cd8134060acf6ffe8d81215c394d800c
(cherry picked from commit b2f875bc)