RFCOMM: Add more logging on failure cases

* Log every instance of RFCOMM unexpected disconnection
* Log every time when RFCOMM channel (DLCI) is changed
* Refactor some RFCOMM code to be more readable and easier for logging
* Remove excessive logging in power manager
* Assign HSP default version more appropriate location
* Replace some usage of NULL with nullptr

Bug: 77224743
Test: make, no functional changes
Change-Id: I5e62dd6d338f7e92a8b8d39c1fe155b71396034c