Cache the connectTime when merging calls into a conference.

Set the connect time for each conference participant on participant's
change, so the newly created ConferenceParticipantConnection has its own
connect time. Cache the connectTime when merging calls into a conference
up to 1 minute.

Bug: 120029844
Test: Manual
Change-Id: I2b791e6175470a62ae70005b8adcb5f5cb6abeb4