Bluetooth: Host: Ensure only connected peers affect `_bt_gatt_ccc.value`

The doc on `_bt_gatt_ccc.value` specifies that only connected peers
contribute to that value. But before this change, it was computed from
all entries in `_bt_gatt_ccc.cfg`, which include bonded but not
connected peers when `CONFIG_BT_SETTINGS_CCC_LAZY_LOADING` is set.

Co-authored-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>
Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>