Bluetooth: Classic: Move classic functions from conn.c to conn_br.c

Move functions `bt_sco_cleanup()`, `bt_conn_create_br()`,
`bt_hci_connect_br_cancel()`, and `bt_conn_get_dst_br()` from `conn.c`
to `conn_br.c`.

Change the function `bt_hci_connect_br_cancel()` to internal public
function.

Add a header file `conn_br_internal.h`. And declare the function
`bt_hci_connect_br_cancel()` in the header file.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>