kmswebrtcendpoint: use getters instead of access directly BaseSdp attrs
kmswebrtcendpoint: use getters instead of access directly BaseSdp attrs

Change-Id: If918f33ca0da0df58f5347ca2aa3c4953b39800a

TribeMedia/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements,TribeMedia/kms-elements,shelsonjava/kms-elements