L2CAP: Use new event stream model

Move event handling logic to client code for flexibility. Instead of
having multiple streams and requirements for subscribing and
unsubscribing a stream, server pushes all events to the stream, and let
the client fetch all of them at the same time. Adding an event handler
in client code which allows callbacks upon receiving an event.

Rewrote the existing L2CAP cert tests and improved the stability.

Next step is to migrate this new event stream model to other modules and
facade code.

Also handle command timeout properly (L2CAP/COS/CED/BV-08-C).

Bug: 144305062
Bug: 141557006
Test: cert/run_cert.sh
Change-Id: I2bea1c7c29ce941e1c357273cca853a665c67baa