L2CAP: Fix and add unit tests for fixed channel acquire and release

* Fixed a comparison bug in referece counting
* Add unit test to verify that Acquire() will hold an ACL connection
beyond idle timeout limit
* Add unit test to verify that Release() will schedule idle timeout
disconnect

Bug: 138261142
Test: bluetooth_test_gd
Change-Id: I79ca2d641f321226a1ccff191b51351d73fafb78