Maintain call time across SRVCC (2/3).

When an IMS conference call encounters an SRVCC event requiring a
fallback to GSM/CDMA, we need to maintain the call time for the
conference host across this fallback. So, adding a connectTime attribute
in the underlying conference host's connection object which can be used
to account for the call time from the now inactive conference call into
the new call created.

BUG: 21918593
Change-Id: I13e7ae6e31f004e798776c02ac7a6ac09a10f0bd