Set RFCOMM SO_SNDBUF size to 70 KB for large RFCOMM writes.

With a 64 KB OBEX MTU, net/rfcomm/sock.c:rfcomm_sock_sendmsg() quietly drops data.

The default SO_SNDBUF is 24 KB. Empircally, 36 KB still drops, and 38 KB no longer drops (this is because SO_SNDBUF is doubled in net/core/sock.c and then there is OBEX/RFCOMM overhead). Set to 70 KB so we have plenty of room to spare.

See http://b/2090000 to investigate this in more detail later.