Add Disconnecting State to HeadsetClientState Machine

HeadsetClientStateMachine could potentially enter a state where we do
not receive a termination event from remote when attempting to
disconnect a call. This was fixed by adding a Disconnecting state to the state machine and
including a DISCONNECT_TIMEOUT case.

Tag: #stability
Bug: 235893458
Bug: 366041766
Test: atest HeadsetClienStateMachinTest
Flag: com.android.bluetooth.flags.hfp_client_disconnecting_state
Change-Id: Ic8c22f200c180a1e50fc9cfabd687021f5a8a8ed