Change conference connect time to ignore undefined connect times.

- The existing code to get the earliest connect time for a conference
could potentially return 0 (i.e. undefined).  Although I've not run into
this in my own testing, it is theoretically possible.
I suspect the reason that the connect time was being reset when starting
a conference is that one of the ImsCalls must be returning a connect time
of zero, which in Telecom is treated as "unspecified" (in other words,
no connect time specified, so use now).

Bug: 22089306
Change-Id: I79be11b67767650455600befbecf841ff8b87c09