HFP: cleanup sco_idx properly while receive SCO SHUTDOWN event (2/2)

In below corner cases, sco_idx of scb is not cleanup so that the
following create sco connection failed with sco index already in use.

1) Rfcomm closed before SCO is disconnected, then bta_ag_sco_conn_close
would be ignored because peer address is empty.
2) Receive SCO_SHUTDOWN or SCO_CONN_CLOSE event while SCO SM is SHUTTING
state, TWSP path will not clean sco_idx.

Change-Id: I92fd78273f658033d4b31e6e25db7c422c9714ea
CRs-Fixed: 2643045