ACL packet recombination

Recombine ACL fragments according to indicated L2CAP PDU size. Used the
workaround that rootcanal always sends the same ACL packet to remote to
verify.

Also added ACL fragmentation support in rootcanal. Recombination in
rootcanal is not supported so far, so whenever we want to send an ACL
packet fragment, we never buffer it until we have a complete L2CAP PDU.

Test: cert/run_cert_facade_only.sh run_cert.sh
Change-Id: Ieb4d3c987327867a39ce210474b84645175b8984