sip: handle media-level ice-ufrag/ice-pwd

as per RFC-8839 describe in section 5.4:

    The "ice-pwd" and "ice-ufrag" attributes can appear
    at either the session-level or media-level.

This seems to be the case on Asterisk servers with ICE enabled (no
ICE at session-level, only in media-level). Note that technically
we can have a SDP with multiple ufrag/pwd and multiple ICE sessions,
but this would need other changes. For now, we can at least handle
the case were ufrag/pwd is at media-level.

Change-Id: If92d8e6194fac2b71185ddf934a6c3a78e3463c0