L2cap SignallingManager fix and improvement

* Fix SignallingManager to send configuration request after remote
feature is received.
* Merge duplicated code when we send configuration request to a helper
function.
* Correctly queue disconnection request.
* In L2capTest, explicitly send NO_FCS in configuration request and
response from CERT, if we don't want to use FCS. By default it's on.

Test: cert/run --host
Change-Id: Ida8474d27df1f90c026d7a90ce40e13a550bb8dd