Map CODE_LOCAL_ENDED_BY_CONFERENCE_MERGE to NORMAL disconnect cause

This change also optimizes imports

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: I7ea843683eebfa2e60683ddffd11995b426f0a12