libstagefright: poke the udp hole twice for future incoming udp packets

- only firing off udp packets once to the source address or the endpoint
  could still be limited in some NAT network environment. As a result,
  client can't receive rtp packets over udp.

- send udp packets to both of the two addresses if they are available.

Change-Id: I7e8a0b36a82bcd85b307c9c15dcfee2bcd7e6030