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
Merged-In: Ic3b6b0350b4e6a321d175e001df2f786a2317e60
Change-Id: Ic3b6b0350b4e6a321d175e001df2f786a2317e60
(cherry picked from commit e3de76f3)