HFP: Fix regressions in HFP state machine

* When we reject an incoming HF connection, we should notify the event
  to other parts of the stack so that hanlders such as PhonePolicy could
  react to the event such as attempting A2DP connection
* During SLC establishment, when remote device sends an unexpected
  command, we should still try to handle them, but generate a warning
  message to help debugging

Bug: 65246419
Test: connect with car kits and headsets
Change-Id: I51f861a11948e5bfe3a0e0a58cee072f1eef4f5c