res_rtp_asterisk: Only learn a new source in learn state.

This change moves the logic which learns a new source address
for RTP so it only occurs in the learning state. The learning
state is entered on initial allocation of RTP or if we are
told that the remote address for the media has changed. While
in the learning state if we continue to receive media from
the original source we restart the learning process. It is
only once we receive a sufficient number of RTP packets from
the new source that we will switch to it. Once this is done
the closed state is entered where all packets that do not
originate from the expected source are dropped.

The learning process has also been improved to take into
account the time between received packets so a flood of them
while in the learning state does not cause media to be switched.

Finally RTCP now drops packets which are not for the learned
SSRC if strict RTP is enabled.

ASTERISK-27013

Change-Id: I56a96e993700906355e79bc880ad9d4ad3ab129c