kmsrtpendpoint: use getters instead of access directly BaseSdp attrs

Change-Id: Ifebb67e38aa7380886f629a3d6e3c8b2dd7cdbce
kmswebrtcendpoint: use getters instead of access directly BaseSdp attrs

Change-Id: If918f33ca0da0df58f5347ca2aa3c4953b39800a