res_rtp_asterisk: Fix packet stats on bridged connection

rxcount, txcount, rxoctetcount and txoctetcount weren't being calculated
for bridged streams because the calulations were being done after the
bridged short-circuit.  Actually, rxoctetcount wasn't ever being calculated.

Moved the calculations so they occur for all valid received packets and
all transmitted packets.  Also added rxoctetcount and txoctetcount to
ast_rtp_instance_stat.

Change-Id: I08fb06011a82d38c3b4068867a615068fbe59cbb