L2CAP: Allow opening connection, and add PTS request_connection test

* Allow reserving a channel to send channel connection request
* Handle open channel request from user API
* Add PTS test_basic_operation_request_connection
(L2CAP/COS/CED/BV-01-C)

Test: run_cert_test
Bug: 141557006
Change-Id: Ia8245a3f944dd1e731808d1c99ac79047b9d67a8