Merge changes I83637165,I4af96edd,I8e0ef282,I6bd6f659,I25e6810c, ...

* changes:
  Wrap l2c::IsBonding in API
  Streamline includes stack/l2cap/l2c_link
  Move l2cu_get_next_buffer_to_send
  Move stack/l2cap/::l2cu_get_next_channel_in_rr
  Remove def L2CAP_ROUND_ROBIN_CHANNEL_SERVICE
  Remove def L2CAP_NUM_FIXED_CHNLS Optimization