Propogate EVENT_CALL_MERGE_FAILED Connection Event

Propogate the EVENT_CALL_MERGE_FAILED Connection Event to the InCall UI
when setConferenceMergeFailed has been received from the Connection.

Also, a test was created to verify the Connection Event propogates to
the InCallService

Bug: 28847375
Change-Id: Id0bd9ca68528d442a658dfb4df8da20c1344e8da