Fix a race condition during HCI module shutdown

If an HCI event (e.g., incoming event) happens during
HCI module shutdown, and the processing of that event
calls update_command_response_timer() right after
"alarm_free(command_response_timer)" in the beginning
of hci_module_shut_down(), then update_command_response_timer()
will try to schedule an alarm on command_response_timer that is NULL.

Test: Enable/disable Bluetooth, added unit tests
Bug: 37241256
Change-Id: I7fd58446caedc135e906953644201f88b5df32bb