Add new reason code for IMS Call ended during conference merge process

Test: Following scenarios for VoLTE and CS call and
      verified the cause sent in PreciseCallState.

 1) Conference call merge
 2) Conference call, one remote party ends the call
 3) Conference call, all remote parties end the call
 4) Conference call, hangup locally
 5) Normal call and hangup locally
 6) Normal call and remote party ends the call

Bug: 38225047
Change-Id: I36edb0cbff4b130b0942a46dc6c15a7d842789c2