HFP: Ignore DISCONNECTED state in PhoneStateChange

* DISCONNECTED state is a transient state from Telecom service and will
  always lead to IDLE state when the call is completely disconnected
* This state should be ignored as HFP specification does not handle
  cases when a call is being disconnected

Bug: 78674822
Test: Receive incoming call and hangup
Change-Id: Icebdf9b5fdffad3e2b78645d4f0c69ec13b2604c