SDP: Invoke complete callback when L2cap channel is closed

Some remote devices will reject the second SDP connection if the
previous one is not fully closed.  We should wait until we receive the
disconnection response from the remote before we invoke callback.

Bug: 197980126
Tag: #compatibility
Test: test with the remote car kit
Change-Id: Ic0b1ceea8a1bd17b220bdc52ea404edf9428d3d2