Clean up BLE connection state state machine

* get rid of BLE_DIR_CONN - we never go into this state any more
* BLE_BG_CONN -> BLE_CONNECTING
* use btm_ble_get_conn_st instead of directly accessing control block

Test: compilation
Bug: 112827989
Change-Id: I4aa09a8b558c6f2161806a6d27bee0560fd19fd8