invoke bta_av_sco_chg_cback when SCO goes from SHUTTING to SHUTDOWN state

When BTA_AG_SCO_OPEN_ST transits to BTA_AG_SCO_SHUTTING_ST, the bta_av_sco_chg_cback may not report # of sco links on time.
Add another bta_av_sco_chg_cback check when BTA_AG_SCO_SHUTTING_ST goes to BTA_AG_SCO_SHUTDOWN_ST.

Bug: 234976097
Change-Id: I2e370facd0e01054645d018a663e41e2401a1f0f
Test: manual 

system/bta/ag/bta_ag_sco.cc

@@ -1104,6 +1104,7 @@ static void bta_ag_sco_event(tBTA_AG_SCB* p_scb, uint8_t event) {
           /* If last SCO instance then finish shutting down */
           if (!bta_ag_other_scb_open(p_scb)) {
             p_sco->state = BTA_AG_SCO_SHUTDOWN_ST;
+            bta_sys_sco_unuse(BTA_ID_AG, p_scb->app_id, p_scb->peer_addr);
           } else /* Other instance is still listening */
           {
             p_sco->state = BTA_AG_SCO_LISTEN_ST;