res_rtp_asterisk: Fix ICE candidate nomination

res_rtp_asterisk: Fix ICE candidate nomination

If the ICE role is not set right away, we might have a role conflict
that stays undetected and ICE finishing with successful tests and no
candidate nominated. This was introduced by ASTERISK-27088.

To avoid this, we set the role as soon as before but only if the ICE
state permits it: still checking and not yet nominating candidates or
completed.

ASTERISK-27646

Change-Id: I5dbc69ad63cacbb067922850fbb113d479bd729c