Fixing conference merge where only one party is added to conference.

- added "swap calls" flag to the onCallMerged listener.  This is used
to indicate that the fg/bg calls need to be swapped after the merge.
- changed how we get the connect time for the conference.  This is now
stored when a conference merge is started (required because of the delaying
of merge complete; one of the calls is already non-existent at the time
we want to get the connect time).


Bug: 18960042
Change-Id: Icc4b60756d333373fe181612824b0c13c076408b