channel.c: Move setting RTP stats from ast_softhangup to ast_ari_channels_hangup.

channel.c: Move setting RTP stats from ast_softhangup to ast_ari_channels_hangup.

The original trigger for setting the RTP stats in ast_softhangup() came from
an ARI issue where stats weren't being set in time to be reported on STASIS_END
events. The thought was that setting them in a common place like ast_softhangup()
would ensure the stats were set in possibly other scenarios.  Unfortunately,
setting the RTP stats variables in ast_softhangup() broke ABI as it required
that no channel locks be held which was not the case earlier.

Given that the original issue was ARI, we can move setting the stats to
ast_ari_channels_hangup() in resource_channels just before it calls
ast_softhangup().  This might not catch all cases of the stats not being set,
but it won't break ABI or deadlock either.

Resolves: #1928