bridge_native_rtp: Take the bridge type choice of both channels into account.

The bridge_native_rtp module currently uses the bridge result of the first
channel that joins a bridge as the ultimate result. This means that if the
first channel has direct media enabled but the second does not a direct
media bridge will still occur.

This change makes it so that both sides are taken into account. If either
side forbids the bridge or responds with a local bridge result then
either a generic or local bridge occurs.

ASTERISK-23541 #close
Reported by: Justin E

Review: https://reviewboard.asterisk.org/r/3577/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@414975 f38db490-d61c-443f-a65b-d21fe96a405b