Fix issue that the disconnect tone doesn't sound from BTHS

When the call is disconnected, the BTHF state is changed to IDLE and
SCO connection is disconnected before playing disconnect tone. So the
disconnect tone can't be sound from BTHS.

Add DISCONNECTED state for HFP and keep the state until disconnect tone
sounds from BT.

Test: manual - Checked whether the disconnect tone is sound from the
BTHS
Test: auto - Passed BluetoothPhoneServiceTest
Bug: 69645337

Change-Id: I6deca4ccee97b8b1a50c491810d2b75f6741c0e9