RootCanal: Move link key notification to pairing

* Move link_key_notification to pairing because they are not a part of
  auth and pairing info is required to determine key type later.
* Properly return comb key in legacy pairing

Tag: #refactor
Test: atest liblmp_tests --host
Bug: 235777894
Change-Id: Iaee68ebe5d1525072e99bc1a7a55cfe5865d3a9b