res_rtp_asterisk: Allow remote SSRC to change on an RTP instance.

When SDP renegotiation occurs it is possible for an RTP
instance to be reused for a new stream, resulting in the remote
SSRC changing if it is part of a bundle group. This change
allows this and updates its mapping in the current bundle
group.

ASTERISK-27231

Change-Id: I6e3703974f236bc024c5dbe9bd43adae0c6fb490