Add EVENT_CALL_MERGE_FAILED Connection Event

Currently, if a call merge failure occurs, we can not try again because
the merge button "gets stuck". This change adds a new Connection Event
that notifies the InCall UI to enable the merge button so that the user
can try again.

Bug: 28847375
Change-Id: I376f6ee446e135577a54aa36ff1dfc77af87261a