Rename Conference#setConnectionElapsedTime to clarity.

Change name of conference connection elapsed time method to make it more
clear what it is for.  Updated documentation of this method and its
companion to make it more clear what they are for.

Test: Compile - this is a docs and naming change only.
Bug: 70639525
Change-Id: I02662cb0331cba0d1fe2d4353438a68f334f9192