Signal EVENT_MERGE_COMPLETE connection event on merge completion.

This was previously only sent on merge failure.
We now trigger the merge complete event once all the connections have been
merged (or failed to merge) into the conference.

Test: Manual conference test, verified signal ordering in Telecom logs.
Test: Add unit tests
Bug: 139169804
Change-Id: Ic3b6b0350b4e6a321d175e001df2f786a2317e60