core: Fix client check in unregister

break is misplaced while checking for client in notification hashmap. Should break immediately after a match is found.
shared: Unlock mutex in handle_callback_response

Because of mutex lock, it was not possible to call other api's within a notification callback. Unlocking it to be able to do so.
test: Change test case to handle locking
demo: Added comment clarifying arguments for creating buxton key